4 Speed and 5 Speed Transmissions

Output shaft snap ring (circlip) & groove problem; serial numbers versus model year;
  pawl spring breakage; neutral switches; shift kits; input shaft seal; kickstarter; throw-out
  bearings; shift linkage; bearings, hints on shifting smoothly (preloading the shift lever and why), gear ratios, ETC.
Noisy rattles sounds

Doing it yourself AND recommended repair specialists.

© Copyright, 2013, R. Fleischer

transmission.htm-59A

Many folks have problems understanding, let alone visualizing how a BMW transmission operates.   I am putting a link at the beginning of this article to help you with that understanding....and, a second link to help you understand noises and shifting.

Link #1:  This shows how a typical transmission, in this case a drum type cam similar to a Classic K bike, might shift:    www.gadgetjq.com/transmission.htm   An Airhead shifts similarly except that the shift levers are moved by a flat plate cam.

Link #2:  This shows BMW GS transmission.  Well worthwhile to review this for information on shifting and various noises, etc.   http://micapeak.com/bmw-gs/trans.html
 

Transmission rattling noises
 

This section was added in May, 2013, prompted by an inquiry, and my answer, on the Airheads LIST.  It is being placed near the beginning of this long transmission article, purpose, as this question comes up relatively regularly. It is edited from the original, for clarity.
 

"I noticed a noise the gearbox makes at low idle. I know it is the gearbox because the noise disappears when the clutch is engaged. I can also hear it when putting my ears next to the gearbox when the bike is
on its stand. I am pretty sure this noise existed before but maybe I grew used to it. It is a rattle or clack/clack rotating kind of noise."
 

I'm not there to hear your rattle. Therefore my reply is an 'educated guess'.

It is NORMAL for Airheads to have a gearbox rattle with hot oil, and at idle. It typically does not rattle with cold thick oil. Old airheads were pretty noisy. Worse as various bits and pieces wear. It is not in the slightest a problem of the transmission, USUALLY.

It is typically worse if the carburetors are out of sync, or anything that allows the cylinders to be a bit unbalanced in operation.  This includes irregular ignition timing....rather common on the pre-1979 models. That irregular ignition timing can be seen with an ignition-fired strobe light shining on the timing hole.  There will be double images.  That can come from timing chain sprocket wear, or other associated items, like the chain, guide, etc.

The cam that operates the valves is a jerky load on the timing chain...that in itself can cause irregular power pulses, primarily but not exclusively from irregular ignition pulses. A bent cam tip, even as little as 0.001", can cause irregular ignition timing and is not unusual. The irregular power pulses cause the engine to not rotate smoothly, that causes jerkiness on parts in the transmission, causing the "Airhead Rattle".
The tell-tale sign is that the noise goes totally away when you pull-in the clutch lever at the handlebars and, typically, tends to go away if you raise the idle rpm with the throttle just a bit.

Do NOT have the engine idling too slow. Some books may show as low as 800 rpm. Back in the old days of heavy flywheels, especially on the R60 model, it was common for folks to brag about how smooth and silent their engine was at idle.  UNfortunately, trying to idle the engine so low (many would try for 600 rpm, let alone 800), is BAD for the engine.    

I highly suggest 1025 rpm. If the idle is too slow, oiling to the chain and sprockets will likely be low enough to accelerate wear on those items. The only oil they get is from the hole in the oil pressure regulator, and if oil pressure is low, there is no oiling.  The pressure regulator needs 75 psi or more to operate. Thin oil, worn bearings, etc....contribute to low oil pressure, as does low rpm as that means lower rpm on the oil pump rotor too.

The timing chain area items are NOT the only items in the engine that will have adverse wear with too low an idling rpm.  The Airhead Rattle is more likely on an older high mileage engine...which has more loss of oil pressure from worn bearings, etc.  Airhead rattle is LESS likely, or less noisy, on models from 1979 due to the improved ignition stability (cam drive to the ignition is much improved), and somewhat better chain tautness control.

It is possible that something else is going on, but the Airhead Rattle is what I suspect.

Use a quality transmission oil. It should be a hypoid GL5 oil, in grade 80W90. Same as used in the
driveshaft and rear drive.

IF you are still worried, wait until the transmission is cold. Have a friend help you if you are
not dexterous. Unscrew the transmission drain bolt. Be prepared to stick a cork or another finger
in the transmission instantly, to avoid loosing more than a tablespoon or two of transmission oil.
This will allow you to not have to purchase another quart or liter of oil...of course, if the oil is
relatively new, you could also just drain it into a clean container, and reuse it.

Put #1 eyeball on the magnetic drain plug. If there are anything more than soft fuzz felt between
your fingers, then there is a problem. NO SHARP PARTICLES NOR PIECES should be felt. If there is
anything small and sharp, you may be safe for a reasonable amount of riding to where the
transmission can be opened. Anything large?....take an in-focus close-up photo, post it in at a
free hosting site on the Internet, then inquire on the LIST, with a link to the photo.

 

The rest of this article begins with a LONG section about 5 speed transmission bearing & circlip problems. 

Transmission problems, checks, and testing: 
What are some simple checks and tests you can do to determine if your 5 speed transmission has a problem developing?

    (1)  AFTER a 10+ mile ride to THOROUGHLY warm up the engine and transmission, in 5th gear
          and maybe 5500 rpm or so, suddenly whack the throttle wide open.  If you feel some vibration
          that is unusual, for SURE you want to do all the tests below, as the forward bearing on the
          output shaft may be disintegrating.  
    (2)  Whether or not test (1) shows anything, jack or otherwise block the rear wheel so it is slightly
          off the ground.  With engine off, in neutral, spin the rear wheel by hand as fast as you can and
          listen to the gearbox. This spins the output shaft bearings only.   NO bearing noises should be
          heard. I prefer to do this with the transmission hot from a ride.            
    (3)  Start the engine (this is with hot engine and transmission) and let it idle in neutral. Pull the
           clutch in for a few seconds and then let it out.   When the clutch then engages, this spins the input
           shaft and cluster shaft bearings only. There should not be a bunch of bearing noise when you
           let the clutch out (you may hear some normal clutch spline chatter).
    (4)  Engine off, transmission in neutral, rotate the rear wheel forward SLOWLY.  This is best
           done with transmission hot from riding.  ZERO roughness and NO notchiness must be felt.
           After that is done, I do recommend you go further, and that is to unbolt the driveshaft from the
           output flange of the transmission and rotate that flange with fingers.  ANY notchiness is cause for
           the transmission to be overhauled.  Now do another test, best done with bolts in the flange, to grab
           onto.   Try to move the flange in and out. ANY free play is likely caused by internal PROBLEMS.

     (5) The transmission output flange has 4 special bolts, and they are NOT to be used with any type of
          lockwasher, contrary to books or what you may be told.  The thread length of the latest PROPER
          bolts are slightly shorter, as the old split lockwasher and longer bolts should be eliminated.
          There is information on this website about that:  Drvshtboltstoolstorque.  I suggest you read that
          article.   The threads should be cleaned, and then a drop of Loctite BLUE applied, and tighten to 29
          foot pounds.  There are various methods of enabling use of a torque wrench here. You can just give
          the bolts a good grunt with a short 12 point wrench; or, torque them properly.
            See ALSO my TOOLS article on this website. 

These are important tests, as usually a problem shows up with these tests, even if there is nothing much on the magnetic drain plug.
            
     (6)  Inspect the transmission drain plug, which has a magnetic center.   If the transmission is
           quite COLD when this is done, and you are quick about it or have three hands and extra fingers to
           plug the hole, you can loose hardly a tablespoon of oil (otherwise, drain and collect it).  
           Inspect the drain plug.  A modest amount of FUZZ, soft-feeling, is fine.  ANY feelable sharp
           particles are cause for further inspection.  NOTE that fairly large amounts of FUZZ,
           soft-feeling, after maybe only a few thousand miles since an oil change (and fuzz removal), CAN
           indicate that the transmission is failing....and for the circlip-less versions, indicate that the 5th gear
           bearing is deteriorating, and the transmission really should be overhauled and the circlip installed.

   
All the above tests are not necessarily 100% conclusive, and later in this article are some
other tests, so do NOT stop reading HERE!


What are some common things that are not usually a transmission failing problem?
 (1)  Small amounts of 'fuzz' on the magnetic drain plug, seen at every scheduled (20K-30K?) gear oil
       change.  The fuzz will NOT have sharp particles.   The fuzz is paste-like, and smooth feeling.
 (2)  Rattling noise from gearbox in neutral, at idle rpm, after thorough warm-up.
 (3)  Shifting problems, especially from 2nd gear downward.  This usually means that your input splines
       need lubrication (unplated early shafts tend to need cleaning and lubrication at 15,000 mile intervals,
       nickel plated shafts at maybe 25K).
 (4)  Shifts not always made.   Check the screw in the shift arm...they are known to loosen.  Use Loctite
       BLUE.

The "circlip":  
The Circlip problem applies TO ONLY SOME 5 speed transmissions.

OVERVIEW of the circlip problem:
From sometime towards the end (??) of the 1984 production year (no longer do I think this began at the beginning of the 1985 production year), BMW's transmission maker made a modification to the transmission.  On the output shaft, they left out a snap ring (circlip) & shortly thereafter (?) they no longer machined the associated output shaft groove (I have reports of at least 3 transmissions from the
1984 model year with grooved shafts but NO circlip, so keep THAT in mind!), located at the nose end.  The shaft part number was not changed.   The no-circlip change caused a lot of grief to owners.  About a decade later the design reverted back to the original reliable version.    A number of these 'circlipless' transmissions have failed, some have had catastrophic failures, ripping the transmission to pieces. 

Here is a link to Anton's website, with photos, and some text, on the circlipless transmission problems...you may find it enlightening!
http://www.largiader.com/articles/circlip/
That is only one of two articles on his website you will find of interest.

http://www.largiader.com/   is Anton's HomePage.
Anton has two articles to look at, not overly clearly shown as two different links, at least not in MY browser, so look on the left side, and find 'transmission, clutch, final drive'.   Put your mouse pointer over "transmission" in "Airhead transmissions and circlip problem".  The word 'transmission' will be seen to be a link to his article. Click on the word.  When finished looking at that article, go back to Anton's HomePage and this time put the mouse pointer over "circlip problem" and click, for the other article.
 

There is not 100% agreement, only maybe 98% (?),  on the exact mode (or reason) for the relatively common failure of later circlipless transmissions.   There are two widely differing basic opinions.  Information here comes from a variety of sources.  Information & food for thought in real detail first appeared in a 2001 Airheads LIST posting by Bob Clement of BMW-Montana, who gave me permission at that time to post his correspondence with me, which I did the majority of, on the LIST.   In the article you
are reading I have added further comments from private communications from several experts in this area, and also my own input.  What follows is information from quite a few sources.  This article in its original initial form, was been submitted & commented upon, & generally approved, by transmission experts.   This article, well, the circlipless information, has since been updated by ME a number of times, and has not been re-submitted, but I believe it to be 100% accurate.

Many private owners have overhauled their own transmissions, some seemingly quite successfully, some using information, tools and parts from Ed Korn or his successor.  Most owners will not want to overhaul a transmission themselves & will entrust it to an expert, as their are some real tricks to making a transmission last a long time and have really smooth operation.  Four of the better known experts are Tom Cutter on the East Coast; Orlando Okleshen, better known as OAK in the Chicago area; Motorwerks on the West Coast (I can't personally recommend them since I don't know their workmanship well enough); Ted Porter (Beemerworks) on the West Coast;  Bob Clement who
does business as BMWMontana; Matt Parkhouse; and Bruno's in Canada (no personal experience with them, but they have a good reputation).   

How to determine if you have one of the possibly troublesome no-circlip transmissions?
This is not so easy, not so cut and dried.     A factory bulletin in 1986 gave no specifics on year and transmission serial number.   There was no change in part number for the output shaft.   THAT is not unusual for BMW, BMW is known to sometimes make a production part change and to use the same part number.    It APPEARS that the transmissions that were affected were shipped with motorcycles of
build date beginning near the end of 1984, so that means that some 1984 models may not have the circlip.  I have had reported to me ONE 1984 circlipless transmission...but this may be faulty, and although requested, I never got the confirming VIN number.   TWO other 1984 transmissions with no circlip but with the same already grooved shaft, were reported to me by this same overhauler, who I trust but I never have received confirming details about numbers and dates.  

There was another, later, factory bulletin, #280, dated 12/08/97, explaining that the circlip (and, therefore the groove) was reinstated, and the SHAFT number was CHANGED. HOWEVER, it appears that the shaft is actually the same as the 5 speed output shafts built from 1974 into 1984.   The specified 'new' shaft is 23-21-1-338-793.   BMW raised the price of this shaft tremendously.  One can, and competent transmission overhaulers DO, modify the non-circlip shaft, but this needs to be done very carefully.   The bulletin also mentioned a 'special bearing' for the front of  the output shaft.  There is some controversy about this, and this bearing was made by a Japanese bearing manufacturer.  More later herein.

Transmissions beginning with serial 240765 SUPPOSEDLY had the circlip re-installed.    Confusingly, no year was specified, but it appears to be mid or late 1995.    Further confusion exists... as year of production and transmission serial number may well not go hand-in-hand.  I have obtained information on model year versus transmission serial numbers, and they are presented later in this article.   
It is my belief that you cannot DEPEND on even a 1995 bike as having the circlip, due to an unknown transmission manufacturing date; AND, sometimes later dated transmissions in 1995 did not have the circlip; see my list of reported transmissions, much later in this article.  You probably also can not DEPEND on a 1984 to have, or not have, the circlip. The best information will be had by looking at the transmission serial number.   Even THAT is sometimes questionable, if you look at the chart later on this page.   However, it seems like 1984 transmissions are likely going to be OK.

Summing up: ...it is possible for a transmission built from 1984, up until AND AFTER transmission serial number 240765, to not have the circlip and the shaft has or has not a groove for that circlip.   We have had reports of transmissions AFTER that # NOT having the groove nor the circlip!!...and at least one is listed below.

How do you find your transmission serial number, and what range of serials can you expect??  Transmission serial numbers are found in one of three places.  Early transmissions, from 1974 to 1981 (or, I believe, as late as late 1983, depending on country the motorcycle was shipped to), will have the serial number centered on the top rear center or top front center, where you can not see it without removing the air cleaner, etc.   Thereafter, the serial number is located at the top area of the left side, just barely BELOW where the left airbox outlet hose connects; so you have to squat down to see it.   Serials are stamped into the aluminum transmission case.  There are some variances, and so noted below.

When reading in this article about 'year', be advised that BMW's 'model year' includes motorcycles that were built towards the end of the prior year, and it is quite possible to see, for instance, a build date of September, to be included in the following year's model.    This is due to BMW policy of closing the plants for the annual month-long holiday (vacation). An October, November, December production can be the NEXT YEAR'S BIKE.   Take a look at the list of transmission numbers well below in the NEXT LOWER section, not what is JUST BELOW.

1974:   serial numbers ranged from Y-4300 -> Y20050; after which the numbers continued withOUT
           the Y letter.  Serial numbers are located at top rear, at center of case.
1975:   information sketchy, some end of 1974 transmissions probably used, may have Y prefixes;
           otherwise no Y. Numbers from 4225 -> 13500 have been identified, without the Y prefix. 
           Serial number same place as 1974.
1976:   No letter.  5 digit serial numbers.  Probably uses serial numbers 25000 through 51000,
           some of these may be in 1977 models.
1977:   Some from 1976 used, so one can expect serial numbers from 46000 to 63000.  Serial
           numbers either at top rear center or top front center, in front portion of casting.
1978:   72400 -> 83000, and after those had a prefix letter Z which may have been early 1979
           transmissions made in late 1978, as Z-0870 -> Z-0940.  Serials are now on the FRONT inside
           face of the casting.
1979:   All have Z number prefix, from Z-19100 -> Z-36750  (at least).

NOTE 1:  Beginning in 79 the gusset reinforcements at the bottom of the case were cross hatch like a crossword puzzle. The 78's had gusset reinforcement running only from front to rear. Left to right did not exist.

NOTE 2: BMW has had fun and games with transmission serial numbers.  It is possible that early transmissions WITH kickstarters had ZSA serials, for one example.

1980:   The Z number now begins with a zero:  Z-052800 -> Z-064950

From 1981, for awhile at least to 1982, things get a bit confused, with serial numbers LOWER .
1981:   There are some numbers that seem to fall in the 1980 group, but the casting is different. 
            Most will find that the serial number is now on the left exterior side, top rear.  Expect serial
            numbers of Z-006111 -> Z-029900
 

In mid or later 1981 (this is unclear to me, although the serial number of the transmission is known, see just below), and some think it happened fully by mid-1982, BMW installed the so-called 'shift kit'  inside the transmission.   This is a fairly extensive kit with a revised cam shape, modified shifter arm, etc.  It is retrofitable, and can be considered for earlier transmissions when overhauled.   BMW has a habit of phasing in changes, sometimes on some models long before others....and on occasion one might find a far later serial number withOUT the shift kit change; and transmissions with partial changes.   As has been noted well above, the 1984+ transmissions had the transmission serial number on the left outside, just under the air-box fitting surface, just below the air tube to the left carburetor area.   The earlier transmissions had the serial number at either the rear top, or front top, but you must pull the air-box to see the number.  You will do that at the spline service anyway.    The serial number for the beginning of the shift kit installation is:   56477 (if one can believe that). The prefix was Z or ZSA.   Yes, this information seems to conflict with the charting of transmission numbers here!

1982:   Z-036600 -> Z-060400
1983:   Z-074700 -> Z-084299
Beginning in very late 1983, or possibly in early 1984, the serial number is located on the left side, just below the aircleaner outlet to the left carburetor.
1984:   Z-084339 -> Z-104600
1985:   Z-113701 -> Z-130150
1986:   ZSA-125500 -> Z-125600     
Note:   BMW seems to be using, at least a fair amount of time, the letters ZSA for kickstart transmissions.   More information will be provided to clarify this, in the future, if I can obtain it.

1987:   
1988:   Z letter is dropped (tentative information for exact date this happened).    From whatever
           time in 1988, serial numbers all now have a 3 letter suffix.  Expect numbers from 0147440
           AAB -> 0164300AAI.   Suffix's may, however, be AAB, AAI, AAJ, ETC.
1989:   0154140 -> 0176330 and suffix   (note from snowbum:  I am SURE that the serials go higher
           in the 0176xxx)
1990:   0180939 and suffix
1991:   0190460 -> 0198650, and suffix
1992:   0207050 -> 0215650, and suffix
1993:   0204190 -> 0230075, and suffix
1994:   0237930 -> 0238660, and suffix
1995:   0236539 -> 0254340, and suffix

Some transmissions, by their serial number, may have been put into motorcycles identified by VIN numbers as from a year out of the irregular sequence, above.   Example:  your bike is a late mfr'd 1991, and has a transmission serial of 0204xxx plus some 3 character suffix noted in the 1988+ year, above.
 

  I am collecting information on bikes with KNOWN circlip-less transmissions....THAT MEANS...known BY DISASSEMBLY. Note that I am also listing some 1995 production with and without circlips.

 Reported to ME circlipless transmissions are:

1984: ONE, so far reported:   1984 R80RT....Never have received the VIN number and thus the bike manufacturing date.   Transmission is Z016199.  Strange number!  Transmission appeared to be original.   It HAD the grooved shaft. NO CIRCLIP.  Well-known transmission overhauler (not sure who that was now) had seen three total like this.  Strange, ...as this serial number would appear to be for a much earlier bike. It is possible that this was a transmission that was from 1981, had been serviced earlier by
someone, perhaps a dealership; who left it that way.  NO information, maybe never.

The bottom line, SO FAR, is that NO PROVEN 1984 year production has been reported to me that is missing a circlip.

1985 R80RT, transmission Z113701, VIN WB1046401F6490338, production 2/1985
1985 R80, transmission Z115902, VIN WB104630XF6480439, production 3/1985

1986 R65, transmission Z127619, ECE (Australian model bike). NO circlip, NO groove, production 12/85.
1986 R80GS, transmission ZSA124393, VIN WB1034805G6363255, production 10/85
1986 R65, transmission Z123469, VIN........................6128105, production 9/1985
1986 R80, transmission Z125576, VIN WB1046303G6480655, production 10/1985
1986 R80GS, transmission  # unknown, VIN WB1034801G6363284, production 10/1985
1986 R80, transmission Z128362, VIN WB104630XG6480782, production 12/1985

1987 R80RT, transmission Z138188, VIN WB1046407H6491304, production 10/1986
1987 R80 USA, transmission 0144961AAB,  VIN WB1046304H6481119, production 05/1987
1987 R80, transmission 0138910AAB, VIN WB1046301H6481093, production 11/1986

1988 R100GS, transmission 0151096AAI, VIN WB104780XJ6152090, production 11/1987
1988 R100GS, transmission 0155023AAI, VIN WB104780XJ6152610, production 02/1988
1988 R100GS, transmission 0154855AAI, VIN WB1047801J6152611, production 02/1988
1988 R100GS, transmission 0156870AAI, VIN WB1047803J6152688, production 03/1988
1988 R100GS, transmission 0155282AAi, VIN WB1047304J6277710, production 02/1988
        NO circlip, NO groove

1988 R80 monolever, transmission 0161951AAB, bike serial number 6448037, production 09/1988
1988 R100RS, USA model, transmission 0154894AAB, VIN WB1046600J6247481, production 02/1988
1988 R100RT, USA model, transmission 0162792AAB, VIN WB1046907K6293354, production 10/1988

1989 R100GS, transmission 0163629AAI, VIN WB1047809K6153197, production 10/1988
1989 R100GS, transmission 0164105AAI, VIN WB1047302K6332169, production 11/1988
1989 R100PD, transmission 0171806AA1, VIN WB1047905L6134016, production 06/1989
1989 R80GS, transmission 0176603AAI,  frame serial 6249769, production 11/89.  No circlip, NO groove.

1990 R100RT, transmission 0180939AAB, VIN WB1046902L6293473, production 04/1990

1991 R100RT, transmission 0185431AAB, VIN WB1046908M6293561, production 08/1990
1991 R100GS, transmission 01292165AA1, VIN WB1048803M0230223, production 02/1991

1991 R100R, transmission 0204599 AAI, VIN WB1048707N0280111, production 10/1991, actually a
        1992 model

1992 R100R, transmission 0204181AAI,  VIN  WB1048706N0280049, production 10/1991
1992 R100GS, transmission                    ,  VIN  WB1048803N0230711, production 10/1991
1992 (R100GS??), transmission 01213536,   VIN ??                                   1992??
1992 (??) R100GSPD, transmission 0207039AAI, VIN not available, can't confirm production year w/o it.
1992 R100GS/PD, transmission 0209970AA1, VIN WB1048909N0047487, production 01/1992

1993 R100GS, transmission 022756AA1, VIN WB1048801P0231259, production 09/1992
1993 R100GS, transmission 0224420AA1, VIN WB1048804P0231319, production 10/1992
1993 R100GS, transmission 0226691AA1, VIN WB1048304P6467330, production 12/1992
1993 R100GS, transmission 0228652 AAJ, VIN .....646450, production 01/1993

1994 R100R,  transmission 023696AA1,  VIN  WB1048709R0280858, production 01/1994
1994 R100R, transmission 238655AA1
1994 R100GS, transmission 0237931AA1, lug stamped NI
1994 R100GS, transmission 0238984AA1, VIN WB1048805R0231610, production 01/1994

1995 R100R Classic, Euro version (also known as R100-R91), serial 6469362, production 04/1995, with transmission 0249173AAI.   Had groove, had circlip, was original, never before opened.

Here is a LATER production, and later serial OF TRANSMISSION, withOUT groove and circlip:
1995 R100 Mystic, transmission 0251758AA1, VIN WB1049706S0400255, production 09/1995, had no groove, no circlip.   Because of the production date, I removed my prior comments about possibly only early 1995 models not having the circlip.

  More on Testing:

Have the rear wheel slightly off the ground, transmission in neutral, and rotate the rear wheel slowly.  No notchiness should be felt.  This test can be done with the transmission cold, but is a bit more revealing if done just after a 10 mile+ ride, so the oil is hot.    This test tends to also show up a bad bearing caused by allowing water to get into the transmission, usually from over-vigorous spraying during washing (at the hollow speedometer cable bolt), or from a bad speedometer cable rubber boot (very common problem, both water problems can lead to $$$ repairs).    This test also tends to show up bad driveshaft u-joints on the Paralever models, and disconnecting the U-joint at the transmission output flange, and rotating that flange, will allow a yes or no on U-joint and transmission.   NOTE!....I highly recommend making sure that your speedometer cable boot fits snugly, and is either internally stuffed with a NON-MELTING grease; or,
BETTER:  sealed at the top with silicone RTV;  if the top is poor, water can run down into the transmission, causing $$$ damage.    Milky (coffee with cream) look to transmission oil is VERY BAD to have happen...it destroys bearings.

For the problem under discussion, the circlip-less transmissions, the most important indication of a SERIOUS problem that you may have is sudden VIBRATION....and possibly noises.    If you feel an unusual vibration, and it need NOT be suddenly extreme at all, not even very strong at all, and determine that it is transmission related (pull in the clutch at a stop, engine running....try in gear and also in neutral), that is the time to stop, right then, and have the bike towed.  Failure to comply will likely, within a few miles, lead to a massive failure.  I CANNOT EMPHASIZE THIS ENOUGH, FAILURE TO STOP RIDING, NOW....NOT 50 MILES LATER AT THE NEAREST DEALERSHIP OR REPAIR SHOP,,,, CAN COST YOU A BUNCH MORE MONEY.

If your transmission is in the range of the circlip-less ones, you might SERIOUSLY consider an overhaul well before you have a failure, as the number of $$ parts to be replaced always greatly increases as you let the transmission accumulate mileage.

Even if you hear and feel nothing peculiar; every few thousand miles, put the bike on the center-stand, when the engine and transmission are thoroughly warmed up from a ride.  With engine off, spin the rear wheel and listen for growly sounds.  Turn the wheel slowly and feel for notchiness.  When the engine and transmission have cooled overnight, check the drain plug.  GENERALLY the degradation is slow, but sometimes it DOES come on suddenly....noises and/or vibration. 

NOTE:  BMW, like most manufacturing companies, is tight-lipped regarding engineering details when it makes a change.  BMW tends to be more tight-lipped than many companies, probably from both a corporate policy and the Germanic 'we don't wrongly engineer ANYthing, WE don't have problems....'.   BMW, Germany, or North America, may well issue Service Bulletins of various sorts, now and then, but these often do NOT spell out details that one might like to have, and sometimes what is said is confusing, especially considering what is not said.   We have to live with that.  Sometimes some of us Wrenches manage to get information that is not very public, to say the least.  In in some instances I have been given information that is almost to be considered Top Secret.  I can NOT divulge in such circumstances my sources...  BUT!!.....one way or the other, in almost every instance, I DO manage to get the information needed to you all.
 

Viewpoints on the circlip 'problem'

#1:  This is the more commonly accepted viewpoint and it is this viewpoint that I personally agree with:
    There are three shafts in the transmission, INPUT, INTERMEDIATE, and OUTPUT.   The front gear teeth on the output shaft is manufactured on an angle, that is, the gear is HELICAL cut.    Its mate must also be that same angle.  Picture in your mind such a pair of mating/meshing gears.   If power is fed to ONE, then the other has forces such as would move that gear along the axis of the shaft, not just being rotated.  This direction of axial force reverses, depending on powering from the engine, or engine braking.  In the 5 speed transmission, even in neutral, the forward gear (5th) gear on the output shaft is being pushed forward any time the clutch is engaged, even in neutral, except in the coasting condition.  A BOSS on the front of that gear pushes against the rear face of the inside race of the front output shaft bearing.  That rear face of the inside race of the front output shaft bearing is the thrust surface for 5th gear.    Thus, 5th gear tries to push against the front bearing.   Another way to look at this, is that forces try to move the helical 5th gear forward, and move the shaft backwards, the gear then moving on the shaft.  Because of this, the front bearing is always damaged, and sometimes the rear bearing, and even the rear cover plate!


There is a tiny amount of end to end float in the shafts fitment in the transmission case (a few thousandths of an inch), established during the shimming process, which ensures that the bearings are not subjected to end-preloading.    Ball bearings, used in this transmission at that point, do NOT like preloading, that makes them heat up, and fail.  

There ARE some very special types of ball bearings designed for end loading, used in such as the worm drive shaft on electric winches, but this is not pertinent here (and those bearings are VERY pricey).    

Prior to the change in what I think was late 1984, there was a hardened snap ring, a CIRCLIP, that fit in a machined groove on that output shaft, it was just forward of the gear, and prevented the bearing from moving forward from that mentioned angled (helical) cut gear pressure.  That kept the pressure from eliminating any decrease in the shimmed float. 

When the circlip is left out, the first thing that happens is that the axial thrust from 5th gear causes the gear to push the inner race of the output bearing along the shaft towards the front of the transmission, as the shaft, in essence, moves rearward and the gear moves forward on it.   If the bearing race moves far enough, it will eliminate any end float.  This places an axial load on the bearing, that will cause it to eventually overheat and fail.  The large front bearing overheats, begins to self-destruct (the cage which keeps the balls in place starts coming apart and metal goes all over the place).  More clearance develops, the output shaft can actually wobble in the bearing.   This will, if bad enough, result in the output flange on the transmission (driveshaft U-joint flange) contacting the transmission lip area where the boot is, and the output rear bearing distorts; and, with metal bits already getting into things, all sorts of mechanical mayhem happens, and rather fast.   From the first sign of unusual vibration, things deteriorate rather quickly.    If allowed to proceed, gears, bearings, shift fork, and even the rear case of the transmission, are ruined.

#2:  This is a far less accepted viewpoint:

This point of view is quite different.  I do NOT agree with this.    This view is that while the helical cut gear does move forward under load, it presses the inner race of the bearing, and it takes that load because the outer ring is against the gearbox case....and thus the circlip is of no matter.   Thus if the clip IS there, the entire shaft moves forward, and the bearing always takes the axial load, and no force moves the inner race relative to the shaft.   This point of view is thus that the smaller bearing should fail first if preload was a problem....and....there are NO circlips on any of the other shafts....and, further, the purpose of the circlip was to assist with disassembly!   During the heating of the transmission, the case should release all bearings, but a circlip-less large bearing COULD remain in the case when the shafts are removed,
and it is a bit of work to remove the bearing...that one, as it is larger, sticks in the case recess... so the clip was used.    These folks agree that the clip removal coincided with the increased bearing failures....but say that the increased failures are NOT caused by the clip removal, it was coincidental with the Paralever introduction.   They say that the GS Paralever introduction, with its extra travel, puts forward thrust
into the gearbox, and even an angular thrust due to the changed design (dual travel angles of the Paralever), and that said angular thrust is taken up by that large 6403 bearing.....and 'proof' is that no extra large REAR bearing was installed by the manufacturer.     Thus, these folks believe EXTERNAL forces are the cause for the gearbox failures, and they tend to blame too high spring preloads and poor lubrication on the splines.   They feel that BMW put the circlip method back into production as it was cheap to do, and shows that BMW 'did something'.

MY explanation:  The above #2 viewpoint is faulty.  Circlip-less gearboxes on NON-Paralever have certainly failed in this area....negating the above arguments.  ALSO, I know of NO failures of any gearboxes in the fashion mentioned in this article that were not fixed permanently by modifying the shaft and installing the circlip (with new bearings, etc....as required).   So...I just can't buy their argument; especially when considering the thrust given by helical gears..   

Note:  It has been reported to me that the stock Paralever driveshaft has exactly the same pivoting length as the swing-arm, assuming the rubber damper is OK, and as long as that is intact, there are no axial forces on the output shaft.  Another reason for negating some of argument #2.

NOTE regarding that previously mentioned 'special 6403-C3 bearing' for the front of the output shaft: 
 It initially, without a very good look, appears similar to any other 6403-C3 bearing, but the front face of the inner race (forward edge of the inner race, the side of the bearing which has writing on it) is cut with a 90° angle (much smaller taper to it than the rear face of the inner race) between the face and the inner hole where the bearing slides over the output shaft instead of the 6403-C3 bearing which has a tapered angle.  The forward taper is almost absent.   Perhaps this was done to minimize the possibility of bearing creep as the bearing is pushed against the retaining circlip by 5th gear.  This modification may well have never been needed.  The regular bearing worked fine, never a problem with bearing creep leading to bearing axial overloading....UNTIL BMW started leaving out the bearing retaining clip.  This new bearing MAY be the accepted choice....but:  The more commonly accepted view is that the elimination of this cheap clip and associated groove caused the problems...AND...I have received word that if one now orders the 'special bearing', you MIGHT get a STANDARD 6404-C3, withOUT the modified inner race face.  The 'special bearing' was used only for awhile, and BMW went back to the regular old bearing???

This is all very confusing, so the part needs inspection.  However, the 6403.C3 bearing, under part number 23-12-1-338-795 may be received by you with this ID on the bearing:  NTN-TM.  It will have a sharp edge on the circlip side, so the tiny ring used with it is not needed.

Generally speaking, modifying the shaft, installing the clip, new bearings, seals, re-shimming, etc., will be very favorably priced, compared to just a new shaft from BMW!     Those doing their own overhauls can have a good machinist cut the groove to accept the snap ring.  Re-shimming is, of course, necessary with new bearings, etc.  
Here are some photos:
 

This is the 5 speed transmission gearset            This is the 'improved' bearing, SAME part number!
note the red line pointing to a circlip.                   note the TM on the bearing.
          

MORE DISCUSSION:
Here is an EDITED (by me) query and reply, as was on the Airheads LIST in November 2004, that will explain about one little thing you might otherwise overlook, if you are overhauling your own transmission:

The transmission was a 1983 or 1984 R100RS.  The owner decided to do an overhaul ...it had been done previously, probably by him, and the mileage was now around 200,000 km.    He noticed a small "rumble" or notchiness in the output when revolving the output flange, the driveshaft shaft disconnected. No sound, no notchiness when tested from the wheel with the driveshaft connected.  A transmission oil change (done every 6 months!) showed normal metal powder on the drain plug magnet.

Transmission was removed and he slightly released the gearbox cover screws (1-2 turns) and the notchy feeling disappeared totally.  Sounds like a preload problem??    The owner then measured the clearance between the output shaft snap ring and the big bearing (6403) inner race; the inner race pushed as far as it goes down the shaft.  A 0.25  mm feeler gauge was a bit tight but a 0.20 mm loose. So, he concluded
that there must be more than  0.20 mm "undefined" clearance in the shaft and it seemed to him that there is no reasonable way to shim the shaft within 0.1 mm as required especially if the bearing may drift along the shaft. The owner then felt that he would have to shim the snap ring and the bearing inner race to zero clearance.  

The owner continued....(my editing here): ""My actual question is about the "bearing drifting along the shaft due to missing snap ring" theory"": 
If the front bearing inner race moves along the shaft (despite press fit) ...what... is holding the shaft in place ...that the drift can take place?   If it is the smaller bearing then the snap ring push will really prevent big bearing drift but this may cause a situation where the shaft drifts in the smaller bearing, not the big one.  Again, a preload will result.  He also wanted to know what happens to the bearing outer races
in operating temperatures? Do they float ie. are they free  to move away from their assembled positions?  The operating temperature is not very far from the assembly temperature where the bearings practically can be dropped in their places.

Tom Cutter replied:  There are several forces at work that can dislodge the bearing inner race from the shaft. One is the weight and forces of  the driveshaft as it undergoes normal rotation, the forces placed upon the output shaft are cyclical, both rotationally and axially.   The bearing is designed to be captivated by the clip to preclude ANY axial movement of the bearing race upon the shaft.  Unfortunately, the
bearing now sold for the output shaft is dimensionally slightly different from the one designed for in the original plan. That bearing had a square corner on the rear face of the inner race, so that it would press evenly upon the circlip. BMW only offered that bearing for a relatively short  period, then they substituted a standard 6403 C3 bearing in its place in the  parts system. The standard 6403 bearing has a VERY
generous radius on the inner  bore, which makes the race bear on the very outer edge of the circlip. In some cases the clip becomes dislodged into the adjacent void. The problem is exactly as you have mentioned, and is the cause of the premature failure of so many gearboxes. The cure is to shim the inner race so that there is no possibility that the bearing can be displaced. This can be done with flat shims, although BMW do not offer such, or by simply fitting the round wire expansion ring that BMW used for the purpose.   (Emphasis in red by me, Snowbum)

This round wire expansion ring sits down in the radius of the bearing bore, and effectively fills the void so that there is metal-to-metal contact from bearing to circlip (snap ring).  The circlip is:  07-11-9-934-100 (size 17 x 1).  This is the same circlip used on the shift shackle in the transmission, in case you get curious about it.

The round wire expansion ring  is 23-21-1-235-006. BMW uses the term Expansion Ring for the wire spacer at the bearing under discussion, and the word circlip or snap ring means the part that goes into the shaft groove.

The output shaft 0.040" (1.01 mm) groove is 0.5 mm deep, located at 17.00 mm from the step on the forward end of the shaft where the 6403 ball bearing sits.  Machining should be by 1.00 mm carbide tool.  Be sure to check your shaft, and these numbers, don't trust ME here that YOUR shaft is the same.  The groove must be VERY precisely located and done so that the round wire expansion ring, mentioned above, CAN be installed.    The inner race of the bearing we have been discussing must be installed so it presses (well, touches) on the circlip.  It should not be installed towards the rear (maximum space between inner race and circlip).   MY feeling is that if things are done precisely, then the expansion ring must be installed.  However....I have mixed feelings on that.
 

I, UNfortunately, never took any of my own photos when doing the circlip machining job.    However, here is a link to an article with photos.  I agree with the article, for the most part, with some hesitancy about BMW not using the round wire expansion ring at times.   The article will show you what the shaft looks like, the groove being cut, etc.
http://www.gunsmoke.com/motorcycling/r100gs/tranny_circlip/index.html
 

Here is another article with photos.    Be sure to look at this one.
http://www.largiader.com/articles/circlip/

http://www.largiader.com/   is Anton's HomePage.
   Anton has two articles to look at, not overly clearly shown as two different links, at least not in MY browser.  Look on the left side, and find 'transmission, clutch, final drive'.   Put your mouse pointer over "transmission" in  "Airhead transmissions and circlip problem".  The word 'transmission' will be seen to be a link to his article. Click on the word.  When finished looking at that article, go back to Anton's HomePage and this time put the mouse pointer over "circlip problem" and click,    for the other article.

One thing to really think about is what to do if you think you have a transmission
withOUT the circlip.  While I am attempting to identify the full range of serial
numbers, year, model, etc., that for sure have no circlips, it still remains a bit
wishy-washy, although less so as time goes on.   You do have a choice on what to
do, or not to do.  

If you intend to go through the transmission yourself, you may be opening a can of worms.  If you send it to a specialist, you will want to contact one of our transmission experts, as there is a LOT to know.  You could send it as a preventative measure.  If things come apart in the transmission, metal will go throughout, and cause a LOT of damage, typically.  
 
 The overhauler's I tend to trust are:

Ted Porter at Beemershop, Scott's Valley, California service@beemershop.com or
tedporter@beemershop.com

Bob Clement in Roberts, Montana   bmwmontana@aol.com

Matt Parkhouse in Colorado Springs   hobomatt@aol.com or bmwhobo@gmail.com

Oak in Chicago area.  askoak@aol.com  Be sure to ask how long the work will take, he may be very backlogged...or have quite doing work.

Anton Largiader in Charlottesville, Virginia   anton@largiader.com
http://www.largiader.com/  
  
Tom Cutter in Yardley, Pennsylvania DBA Rubber Chicken Racing Garage: tpcutter2@aol.com
 

You could also take the attitude that you will watch things carefully, and at the first sign of problems with your OFTEN inspected magnet that is part of the stock drain plug (and, disconnecting driveshaft and rotating the output flange
as noted in #5, way above)....you will pull the transmission and have it modified and overhauled.  You would also carefully be watching for sudden vibration....and have promised yourself to stop right then....not proceed another few miles....

It is up to YOU!!

***Special note on the throwout bearing area.  This is being put into this transmission article (it is expanded upon in the CLUTCH article) due to the potential for someone to think that the transmission has a problem...which, in essence, it does, since the throwout bearing is LOCATED in the rear of the transmission.
Typically, the clutch will start to slip, when the transmission gets fully warmed from riding.

Some tolerances on some transmissions throwout area bore sizes, and throwout pistons, were not held tightly (well, LOOSE enough).  Generally this is thought of as from 1981, when BMW changed the clutch design radically.    If the piston fits into the bore a bit too tightly...it may still operate smooth enough, but under some circumstances (temperature, as in HOT!) may stick.   Measure the piston, and if it is over about 1.13" (28.7 mm), you MAY want to reduce the outside diameter a bit.  I have seen these as large as about 1.142" that still worked OK.  I can't give a hard and fast rule here, but if yours is up to 1.141 or so, I would certainly see how it fits, and if a bit too much friction, I would recommend sanding the OD a bit.  The pressure on them in operation is rather square, so if any doubt, ....if careful... you can chuck the rather short shaft tip end in a drill press and use some rather fine sandpaper for this.  Inspect the bearing, and if it looks bad, replace it.  Grease it with a good LIGHT grease (NO moly). The reason to use light grease is that it takes time and miles for the transmission OIL to reach this throw-out bearing.   Oil the outside of the piston as you assemble this area.  Clean and lubricate the arm and associated parts.  BMW has a replacement piston  23-13-1-464-167 which is pricey as it includes the bearing and is a new design, that eliminates the clearance problem of the old style piston...but you do NOT need to purchase it, unless you cannot find a new old-style bearing (if YOUR bearing is NG), or, you don't wish to sand YOUR old piston (if required).  

NOTE that the original piston is not a one-piece metal part, rather, it is a part-metal-colored-plastic item, and it expands much faster than even the surrounding aluminum alloy, so common 'feel' for clearances may end up leaving it too tight.   The updated part fits directly, no problems.....you measure and decide.  I sometimes test the fit with the transmission at operating temperature.

NOTE!!....There is much more extensive information on the clutch throwout lever, the bearing, etc....be SURE to see it on this website!     clutch.htm
That article has a photo of the later style clutch throwout parts, and the actuating rod, and notes on the FELT on the pre-1981 rods, ETC.   
 

NOTE:   The 4 speed transmission had a good balls-type throwout bearing.  The 5 speed transmission
             had a radial needle bearing.  From 1981, BMW BMW went back to the superior ball bearing.

NOTE:   The 4 speed transmission and the early 5 speed transmission (pre-1981) clutch push rods had
             felts located in a groove, and are installed best from the front.  The 1981+ transmissions did not
             use a felt, and had a SEAL instead, and the rod CAN be installed from the rear.

See the NEUTRAL SWITCH article on how to replace a neutral switch on a 5 speed transmission:   Neutral Switch replacement.

Replacing the input shaft seal:

This can be relatively easy, or can be difficult.  Usually when the seal leaks, the transmission is about ready for an overhaul. Let's assume that is not the case.  If the seal leaks, oil from the transmission can flow along the splined input shaft into the clutch, oiling the clutch, and then it will slip.   When you have the transmission either out (or backwards some), to do the normal scheduled input spline cleaning and re-greasing, put your #1 eyeball on the transmission input seal.  If leaking, remove the transmission to the workbench.  It is important to not score/damage the input shaft when removing the seal.  You can make a tool to try to remove the seal.  HEATING the area with a moderately broad flame of a gas torch will help.  If the seal is not going to come out, the next step is to drill quite a few holes into the seal, the holes should  be perhaps with a 1/32" drill bit.   Drill the holes as close as ou can around at the outer diameter of the seal, but DO NOT drill into the aluminum of the transmission case that is surrounding the seal's outside diameter. I suggest you use a shop vacuum to get all the drilled metal pieces into the vacuum cleaner, not the transmission.   Remove the center of the seal and then try to remove the outer part.   If this does not work, you will have to remove the front cover of the transmission to press out the seal.   Once the seal is out, use a strong solvent and if needed clean up the mounting area very carefully, using fine grit sandpaper if you have to, and use that vacuum cleaner.   When replacing the seal, oil or grease it, otherwise you may dislodge the coiled spring in the seal.   Use some sort of tool such as a socket or big washer, so the seal goes in squarely.

_{NOTE that excessive end play of the transmission input shaft can cause a quite-grabby clutch operation, and that can happen cold and/or hot, but more often when hot.  To fix THAT, one must remove and open the transmission and work on the shaft shimming.  One can get an idea if that is a problem by removing the transmission, heating it to about the temperature of boiling water, and measuring, with a dial indicator, the input shaft end play.  Anything over a few thousandths of an inch is suspect.  I'd not want over maybe 0.005".  This problem with transmissions causing a grabby clutch is not overly common, but it seems to be more so for the 1979 models.  Another cause is excessive end-play on the crankshaft...rare.....about .007" is the maximum I personally would allow, and that is for a DRY assembly, which means you cannot take a measurement that
means much, when oil is in the end parts!}

All the rest of the Transmission "stuff":

Sometimes I am asked about the various years of transmissions as to good points, bad points, etc.    A lot of information is in the article you are reading.   However, the 1974 transmission is a special instance.   This was a transition year for BMW, from the /5 to the /6, and there were some things not so nice about the 1974 transmission.   1974 was the first year for the 5 speed box.  The Pawl springs tended to break.  The kickstart parts were SOFT...and use of the kickstarter is NOT recommended.  There were problems in positively locating the Neutral position, and that wasn't fixed until 1976.  The 1974 transmissions, in other words, did not hold up well.  Some parts are no longer available.  This parts problem extends into the 17.5 and 15 degree parts area (you might find some parts not available that are needed to match your 15 degree parts; so you'd have to convert to 17.5 degree parts)...... AND, when you see the parts prices, you will scream.    If a 1974
transmission is really bad, YOU REALLY MIGHT WANT TO CONSIDER A NEW TRANSMISSION; or, preferably, a really expert rebuilt one (especially one after 1974)...as the price is less, and the quality will be...or can be.... BETTER than a brand-new one!   Quite often, the best thing to do with a really bad 1974 transmission is to obtain a later model; those up to 1980 will install with no problems.  

1974 and 1981 were not good years for the transmissions.
 

Here's some more information:

1. see:    http://www.largiader.com/tech/airtrans/
  That article has a number of photos and descriptions that you may find very useful in understanding the 4 and 5 speed transmissions in a few areas, such as the input gear that has the shock absorber cam ears that sometimes breaks on the early 17.5° transmissions (1979-1982); information on the shifting mechanism and 'shift kit', and some other things.  I highly recommend you review that article....and the links at the bottom. Note that 17.5° gears were supposedly installed from 1982, and I am not at all sure that is correct.

CONTRARY to what Anton says about MY article that you are reading, I cover a huge amount more than just 'historical' data as he put it.  I suspect Anton looked at my article, & did not scan down very far.  I get into more depth on things, including the input cam-gear ears, etc.  Regarding that input gear:  see #3, just a bit below here...for a more vigorous treatment of the input cam gear and the 17.5° transmission, ETC.

2. NOTE!!!  see article:  oiltransfers.htm  The output shaft oil seal on all models except the Paralever, have the open side facing rearward.  The Paralever output shaft oil seal open side faces INward.  

 A new style seal is being used on the Paralevers, install dry, shaped for a couple hours on some sort of mandrel, and then being very careful about the installation, to avoid the seal being damaged by the speedometer drive.  You can use some tape over the drive gear to avoid damaging the seal.  If you have an early model of the Paralever, there may be a goodly sized V-vent at 12 O'Clock in the housing.  Block this vent and drill it 1mm.  Later models have this already modified.   Wet driveshaft models used a green seal. 

If you have a non-Paralever model, and oil is transferring from driveshaft to transmission, you have one or more of:  too high driveshaft oil level; sacked suspension; extreme downhill riding; ....and may want to fill the 12:00 notch....be sure to leave it with a teeny hole in the filling.  You MUST have a hole.  See #4 just below!!

3.   The input gear on the 5 speed transmissions has been changed FOUR times, used with THREE different gearsets.  The original input gear was 23-21-1-231-519, often just called the -519 gear.  This was used from 1974 to sometime in 1982.  The 1979 to 1982 ones have been known to CRACK.  Actually, some from a bit later also have cracked.   For the helical, cluster, and 5th gear, BMW changed, for all by mid-1982 or so, from what is often called the Gear Angle, from 15° to 17-1/2°.  The fiche may show earlier in 1982.   The actual changed angle is not exactly the part of the gear appearance you might think; rather, it was the tooth profile.   Because of this change, the -519 gear also had to be changed. BMW did so, and still had problems with it...so they did another change, beefing it up.   They had to change the input SHAFT too.  This occurred in 04/1982, and continued until other changes, in 02/1985.  The new shaft for the 17.5° gears was 23-21-2-302-331 for NO kickstarter; and -332 for WITH kickstarter.  The shaft was shipped complete, except for rear bearing.  

Further explanation:  Some 1981 bikes had the first changed 17.5° gear on the input shaft....this was the small shaft, with a COARSE spline.   The 1981-2 change was to a FINER spline...and then even later to a larger diameter gear set!    All this being said, still, the generation of 17.5° input gears, from 04/1982 to 02/1985, might break ears.  This is when BMW added splines to the drive dog that mates with the
input gear.  This change occurred with transmission serial number 115167 on 03/1985 (I think).

Here some of the information is presented a bit differently, for clarity:
In April of 1982 (from serial 58225 on the transmission??) the helical-cut gears in the transmission were changed from 15° to 17.5°.  The new 17.5° gears are identified by a cross or star or X marking on one end.   They are NOT interchangeable with the older 15° ones, and only 17.5° matching gearsets can be used. Supposedly BMW also identified these updated transmissions with blue paint dot on the airfilter
housing surface of the transmission.  However, the later transmissions (1984+) do have serial numbers on the outside, check for yours just barely below the air cleaner box on left OUTside.   From 115167 (from April 1985?) the input shaft assembly was redesigned.  The earlier gear had a smaller ID, and fit the smaller input shaft OD...with short splines.  The reason for the second generation update was because the first generation of the 17.5° input shaft gears (4/82>4/85) were weak and the drive ears might break.   These updated parts can be fitted to an earlier gearbox with the 17.5° gears.  The original fragile input gear is probably still available from BMW.  However, the best thing is to install the complete second generation input shaft assembly.   The part is 23-21-2-302-331 if you have no kickstart gear; and 23-21-2-302-332 if you do have the kickstart gear.  Supposedly these transmissions are identified by a black painted panel in the ribs in the aircleaner mounting area.  Note that in one other respect the input shaft used on the 1981+ models is not the same as the older gearboxes, due to the redesign of the
clutch, etc., the earlier ones have a longer nose, see #8 below.  Thus there were at least two changes to the input shaft.

MORE!   It has become apparent to me that some of you do not understand certain things about the 15 to 17.5 degree change by BMW, maybe you have never had a transmission apart, or maybe, for whatever reason, it is still confusing.   I will try to explain it a bit differently here...and add a bit more too.    You must be very careful when changing parts to not mix the wrong gears, and this can easily happen with the bevel-cut 5th gear on the output shaft...yes, the one that can have the circlip area problem.   You already know that the 17.5° gear has an X mark on it.   What may not be clear to you, is that if you need a new intermediate shaft, or have access to the large 20 ton+ type of press needed to disassemble that shaft-set, updating the gear on the intermediate shaft means you MUST change the output shaft bevel gear to the later 17.5° type.  .....AND, must change the -519 gear on the input shaft.    BTW...BMW does not sell the intermediate shaft except as an assembly and it is very pricey; which is why some use old gearboxes for parts!  Early versions of the intermediate shaft are NLA from BMW!

Because of the possibility of you using wrong parts, it is best to inquire on the Airheads LIST if you are at all confused!.........or, let an expert do your transmission job!

4.  Only the 1970 and 1971 transmissions did NOT have the tiny notch for breathing, at the transmission output seal 12:00 position.  (yes, the Paralever boxes generally have the notch sealed or a tiny hole).  Because of that, the driveshafts on the 1970-1971 bikes tend to have their 'rubber' bellows swell up in hot weather riding.  This slight pressurization can also cause oil transfer problems.   An article was written about this, and is in the September 1981 BMW News.  The only GOOD cure is to add the transmission output vent...a drilled hole or hand-filed hole will do.   Other forms of venting, even modifying the 
driveshaft oil plug, etc., do NOT work well (even with a several inch long line run upwards).  The shaft housing could be vented in the more forward area, but the BEST fix, if you have to fix this problem, is to put a vent in the transmission output area, as in later airheads.

5.  There are quite a few 'tricks' and adjustments that don't readily appear to someone taking apart a gearbox.  For a truly good operating gearbox, you WILL want to consider a specialist. 

6A.  The 1974 transmissions had a soft kickstart gear on the input shaft, which can cause problems.  It is best to not use the kickstarter, except in an emergency.    A replacement part will be hardened.

6B.  LOTS of changes were made to the transmissions over the years.  Sometimes the serial numbers of the transmissions, or bike serial number, were not well identified as to when changes were made.  It can get very complicated, one of the reasons I recommend transmission specialists.   Some examples here would include the shifter fork groove on the sliding gears that changed from 6.5 mm to 5.7 mm; square undercut dogs (and associated 'windows') for 3rd, 4th, and 5th gears; the detent spring change on the selector bracket; the casting was changed for a stronger selector fork shaft, old was 100 mm, new was 105 mm.   BMW changed the shifting parts numerous times.  They did NOT incorporate all the changes at the same time.  The OFFSET segment for pawl spring clearance was one such change.  Several changes occurred at that same time, with transmission Z5A79720.

.......and there are DOZENS of these sorts of 'nice to know, or NEEDED to know' items.

6C.  The /6 kickstarters are not really meant to be used a lot.  The 1974 was weak (see 6A.).  NEVER just jump on the kickstart lever.  Be SURE it is properly engaged.  MY method is to use light foot pressure, until the lever seems to meet a mechanical stop, then use the clutch lever at the bars to allow the kickstart lever to move downward an additional small amount.  This ensures proper engagement.  Release the clutch lever. KICK.

NOTE!!   The kickstarter lever on /5 and later will 'bottom' on the footrest rubber.  It is important that it not be a pure metal contact.....and on the /2 bikes, there is a rubber bumper.  The rubber must be intact.

7.  The improved shifting setup, with the external linkage mounted off the footrest is probably still available as a kit, for the earlier 5 speed transmissions.  The design change occurred in 1978.

8.  In 1981 BMW made changes in the transmission clutch throwout bearing area, internally as well as the external shift lever arrangement.  Also in 1981, BMW made a large change to the clutch and flywheel, which became a totally different design, and the flywheel is now called a Clutch Carrier....and the transmission input shaft was changed (shorter) to accommodate those changes.  Transmissions before and after that date can be interchanged if the input shaft is the correct one.     The input shaft is about 24 mm long on the early transmissions, and from 1981, it is about 19 mm long.  It is also possible to shorten the input shaft on an early transmission, in order to fit it to a later clutch unit...by simply using a cutoff disc, etc., even with the transmission still together!    Be sure to radius the forward edge properly.  Within certain guidelines, such as the input shaft length, and internal gears angle cuts, shift kit or not, proper neutral switch...etc...as all mentioned in this page, most all the transmissions generally interchange.

9A.   Shift kit and pawl spring, etc:
Pawl Spring: Inside your transmission is a detent spring, that enables the shifting mechanism to shift gears.  If that spring breaks, ....you are stuck in whatever gear you happen to be in.   You MIGHT be able to remove the fuel tank, turn the bike upside down, and then shift into a gear...maybe.  I've heard of this, never done it myself.  There are 'tools' that will enable you to change the gear the transmission is stuck in, via the oil filler port.  It is tricky, and if you contemplate making or purchasing such a tool, I HIGHLY suggest you look at a transmission with the cover off, while you manipulate that transmission with your 'tool' to see what twisting and turning is needed. 
There is an article on this website about these tools:  Emergency-Shifting-Tool.htm
Any such tool will be rather difficult to use, without practice.

All the 5 speed transmissions up to the early 1980's could have this pawl spring breakage defect.  This particular spring is used in all years all transmissions. 

WHY the breakage?   TWO reasons primarily. The early spring rubs on itself during operation, wearing itself thinner.    The spring may wind too tight on the boss, causing excessive force on the spring. 
Possibly some faulty springs, possibly a few other things; we just do not know.  MUCH MORE on this as this article proceeds.

Pawl spring breakage fixes:  One can  turn down the boss it rides on by about .060".  The boss needs to be around 0.613" or below.  If yours is about .630, then machine it down.  The boss size was probably fixed in production, but exactly when is questionable but as I note later, perhaps in 1975 or all before 1976 model year... or so. Tom Cutter posted to the Airheads list that the original was 15.95-16.0 mm; and the change should be to 15.55-15.6.   I don't hold to such tolerances.  
Here is my take on the matter:
The post the spring is on has had its diameter changed in the 'shift kits'.  You can certainly reduce the diameter of the stock type.  The spring must not bind-up in its operation.   Certain of the so-called 'shift-kit' parts, Pawl 23-31-1-242-892; and Segment (offset link) 23-31-1-231-578, supposedly will 'cure' broken pawl spring problems....but modifications work well. As noted in 6B, above: The OFFSET segment for pawl spring clearance was one of BMW's changes.   What has not been said hardly anyplace, except perhaps Anton Largiader's page, is that the Shift Kit, incorporated by the factory since sometime late in 1981, will keep the pawl engaged, if the spring breaks.

 The shift kit uses spring 23-31-1-242-910, and you must use that spring with the shift kit.  

In mid or later 1981 (this is unclear to me, although the serial number of the transmission is known, see just below), BMW installed the so-called 'shift kit'  inside the transmission.  This is a fairly extensive kit with a revised cam shape, modified shifter arm, etc.  It is retrofitable, and can be considered for earlier transmissions when overhauled.   BMW has a habit of phasing in changes, sometimes on some models long before others....and on occasion one might find a far later serial number withOUT the shift kit change; and transmissions with partial changes.   The 1984+ transmissions had the transmission serial number on the left outside, just under the air-box fitting surface, under the air tube to the left carburetor area.   The earlier transmissions had the serial number at either the rear top, or front top, but you must pull the air-box to see the number.  You will do that at the spline service anyway.    The serial number for the beginning of the shift kit installation is:    56477. The prefix was Z or ZSA.

The purpose of the shift kit was to eliminate false neutrals or hung-up shifting, and improve shifting in several ways; AND, to improve reliability.  The kit is part number 23-31-9-056-150.   The kit as such is no longer available, but the parts ARE.   Note that the shift kit 'fixes' for the false neutrals does not have the same level of improvement in the heavy flywheel models as the later clutch carrier models.

Note as mentioned earlier that numerous parts changes in the shift mechanism occurred over a number of years....such as the offset segments and pawl spring changes.

In the September 2003 issue of BMW Owners News, from page 34, is an ILLUSTRATED article on replacing a broken Pawl Spring in the transmission.  Comments by me:  In illustration 15, the torque obviously should not be 24 foot pounds.....5 to 6 foot pounds is correct.  In the article, the author does not mention that the BOSS needs to be relieved to be sure the spring does not bind up and break, again!  See above in this long transmission article of mine that you are reading, about that boss and spring.  In the November 2003 issue of BMW Owners News, from page 34, is an article on installing the updated shift kit.  There are some errors in this article:
    a.  In the first column of page 34, second paragraph, the kit does NOT NECESSARILY allow
         shifting with a broken spring.
    b.  On page 34, photo #1 text, it is NOT true that there are no differences.   There IS a design
         change.  The 1974 and 1975 had a reverse neutral....that is....the plate PROTRUDED at
         neutral, rather than being DETENTED.  The new design makes finding neutral more distinct
         and positive.   The Neutral switch was changed, and although they look similar, they are
         not.   The newer plate shaft is changed to insure against clashing with the pawl arm.  Photo
         #3 text should not really have the second sentence worded like that.
    c.  On page 35, photo #4 text, it is NOT true that there are no differences.  The new arm has an
         offset to avoid binding.

9B.  Emergency Shifting Tool.   That is a hyperlink!

9C.  The "shift kit" is most effective on the 1981 and 1982 models (to maybe 1983, depends on
exactly when BMW phased in the shift kit themselves, on the various models), because these have the lightened Flywheel, called a Clutch Carrier.   The shift kit might improve earlier transmissions slightly, certainly the pawl spring breakage problem is lessened.  The shift kit will help IF the early heavy flywheel has been lightened.   The shift kit does much less for those Airheads with the heavy flywheels, as far as SHIFTING is concerned. 
The shift kit did NOT come with any instructions.

BE SURE that in your assembly of shift kit parts that the arm is doglegged, that is, off-set.

The shift kit is made up of the following, still available parts:
23-31-1-231-578 segment shaft (offset link)
23-31-1-231-611 shifting cam (for 1-2, 5)
23-31-1-242-892 pawl
23-31-1-242-910 spring
23-31-1-451-563 shifting cam (for 3-4)

***The so-called shift-kit parts, Pawl 23-31-1-242-892; and Segment 23-31-1-231-578, supposedly will 'cure' broken pawl spring problems....but modifications to the stock boss and/or use of the upgraded spring work well for that particular problem.

NOTE, a few paragraphs below, Tom Cutter's comments...and MINE...on how that boss was...or was
not...modified by BMW after 1974 or 1975 (We disagree on that point).

For other views, showing these parts lined up and how the neutral switch works with them, see:
see:    http://www.largiader.com/tech/airtrans/
  That article also has some photos in it that you may find very useful in understanding the 4 and 5 speed transmissions in a few areas.

Use of the shift kit REQUIRES use of the later LONGER neutral switch, or the neutral switch
electrical functions will be in reverse of what they should be!  The later switch is 61-31-1-243-097, and a spacer 61-31-1-355-262 is used.  The 1974-5 neutral switches have a shorter stem.  If you install the wrong switch, it does not work correctly, and may be shifting problems!  

There is are TWO sections in the ElectricalHints.htm article on this website on the neutral switches, neutral lamp, starter circuit, and problems.  It is complex!

Here is something from that article, but please read the entire article:
        ALL 5 speed neutral switches are CLOSED in neutral, turning on the green neutral lamp (enabling the starter function if the starter button is pressed on 5 speed transmissions).   The reason for the shorter and longer neutral switches is that in the early shifting parts, the switch rode on a section of the shifting cam that was a projection, and the switch itself helped make the feel for the 'detent'.  On the revised parts, neutral is much more positively felt, by the switch being in a 'valley' of the shifting cam, hence the switch needs to be longer.  

There is a diode in the neutral circuit.  If that diode shorts, then the lamp is ON if the lever at the bars is pulled.   There is also a peculiarity with the 1978-80 models, which have a master cylinder under the fuel tank.  These incorporate a float switch, whose purpose is to illuminate the brake failure light if the fluid runs low.  The lamp gets tested each time you start the bike, via a diode.  If the diode shorts, and you are also low on fluid, the starter could theoretically energize.

Tom Cutter posted the following to the Airheads E-mailing LIST in September 2003, and it clearly states what the kit also does, and I quote (typos corrected by me) (comments by me, snowbum, are clearly marked in BOLD RED):
"The kit includes a selector arm that has a second rail which will then will allow shifting, albeit a little sloppy, in the event that the spring fails. (note inserted here by snowbum:  I do not believe that to be so, the second arm being designed to prevent overshifts and false neutrals).   In my opinion, when the spring is properly installed and the stress relieved, the failures become non-existent. Nonetheless, the
new arm is a nice fail-safe piece." (see above note by snowbum, who believes the new arm is not a fail-safe piece as such).  Tom later added the following:  "The repairs ...referred to are only necessary if one is retaining the older shift pawl. If using the new shift kit, which I highly recommend be fitted at the same time, the parts are upgraded and don't need modification (Or they are supposed to, I found one old
shift pawl in a shift kit recently). (Snowbum says:  Interesting, if true.   The old pawls were gone decades ago, no one else seems to have found wrong parts in the shift kits).    I am trying to describe this so it will make some sense, but basically, the spring gets over-stressed when it wraps around the boss on the pawl. The boss can be ground to a smaller OD, and the spring attachment point can be modified by grooving the plate, to prevent the spring coil-binding on the boss.  (Snowbum says:  The boss was a problem in the 1974, and maybe some 1975 production, and the oversize boss was fixed no later than sometime in 1975.  I believe that, from then, the springs break from improper heat treatment, or fatigue, or too soft or too hard, maybe brittle....seems to be a difficult part to
manufacture correctly?).    Either or both methods work fine.  The important thing is to assemble the shift plate mechanism, then pull the shifter hook arm back as far as it will go, while looking at the pawl spring. If the spring is binding, it will be apparent. This must be corrected." (snowbum says:  doesn't happen on 1976 and later, but easy to check).

NOTE:   An Airheads LIST inquiry on October 5th, 2011, resulted in Tom Cutter's remarks about the 23-31-1-231-619, the original type of early shift pawl spring, the bent-legged type; replaced by 23-31-1-242-910, with straight legs.  Tom noted that you have to use the dog legged shift segment with it, or the spring will bind.   It is, perhaps, this, that is where/why Tom and I disagree a bit on the subject???

10.   The 4 speed transmission can be a devil to find parts for, and to overhaul correctly.  I strongly suggest going to one of the experts shown below, in particular Oak, Bob Clement, Ted Porter, Tom Cutter and Matt Parkhouse. In fact, those are my recommendations for any BMW transmission. See near the end of this long article a bit on those folks, and how to contact them.

11.  5 speed transmission gear ratios:

12.  There is an additive that works modestly well SOMETIMES to smooth the shifting, particularly with transmissions with the original older style shifting parts.  Do not use this stuff with synthetic oil.  It is Dow Corning M Gear Oil Additive. Comes in quart bottles. Shake well and use up to 2% concentration max, DO NOT USE "UP TO" THE 10%  Dow suggests.  The amount to use is about 18 cc for the
airhead transmission.   This stuff is VERY expensive.  DO NOT use with synthetic oil and do NOT add more than 20 cc!!!!

13.  The 5 speed transmissions have, on the input shaft, a cam and spring shock absorbing system, and one of the gears, that has the cam on one end, is 23-21-1-231-519.  This gear MUST be replaced if it appears to have worn such that it looks even vaguely questionable; the replacement gear from BMW is hardened.  READ the earlier portions of this article on the complexities of changes to the -519
gear....and the input shaft, etc.

14.  There is a plastic roller on a shift lever in the transmission, it is 23-31-1-231-572, and tends, over a long period of time, to start to fail; the steel one from the K models is 23-31-1-451-087, and I have recommended it in the past.  I am not so sure about this, now, as I have heard, ONCE, of the steel K bike roller causing wear on its pin...and against the lever it touches...and perhaps there is some heat treatment variation on that lever...or?     NOTE:  if the roller fails, the transmission may well exhibit jumping out of gear.  This can also happen if the associated spring breaks.   If a question comes up on this point, I defer to the transmission experts mentioned. At least one transmission guru, Ted Porter, is now using a bearing, #688-RS, for that roller.

15.  In 1977 BMW made changes to the transmission.   The slider gears shifting fork grooves were made narrower, now 5.7 mm, previously had been 6.5 mm.  The 3rd, 4th, and 5th gears now had square doglegs (undercut).   The detent spring was changed....and it has 5 turns.  The cam-plate was changed.  The casting was changed for better selector fork shaft support for 3rd and 4th, and the shaft which had been 100 mm is now 105 mm long.

16.  Earlier transmission shift linkage had a foot shift lever connection with a rubber boot over it;  the linkage is NOT adjustable other than the foot peg, which is often found at the lowest position for owners with big boots.  Some install adjustable linkage, or, horrors!, grind away the foot peg casting.  If you install the adjustable linkage from the ST & G/S models, it bolts in place of the bent rod.  23-41-2-301-391.  Drill the hole in the shift lever, as it is just a bit too small in diameter.  Use 1/4" drill.   Add the foam donuts 23-41-7-650-149 over each of the ball joints, or replace your old bad ones.  Keep it all oiled.

Transmissions are best overhauled by an expert with a lot of experience.  For the person who definitely wants to try doing a transmission, you can do it under tutelage or perhaps get Ed Korn's video, ETC.   

Here are some old specifications....there are lots more specifications:

Axial float on all shafts:  0-0.1 mm (0-.004 inch).
Gear change lever shaft axial float on 4 speed: as above.
Fork bolts 17 footpounds.
Output flange nut 160 ftlbs (clean and dry on the tapers!...absolutely NO residual oil!).
End cover nuts 6 ftlbs.
 

The FOUR speed transmission kickstarter idler gear shaft is press-fitted into the rear transmission cover.  There is a tendency, particularly when the gearbox is quite hot from a ride (which expands the aluminum rear cover much more than the steel shaft!), for the shaft to move inwards into the transmission.   Do NOT allow it to move into the transmission!   To do an easy fix is often best, as the formal fix is removing the rear output flange, heating the rear cover, and ...with some experience knowing what to do next, you remove that rear cover, and modify the shaft.   So, the easy fix is to do the modification without gearbox rear cover removed; that is, it is done BEFORE the shaft moves inwards...or worse, the parts inside
fall to the bottom.   The fix involves drilling and tapping the shaft, adding a bolt or screw and large washer.  In one instance I remember from years ago, the shaft had started to move, and the owner drilled the shaft, added a large flat washer and bolt, and heated the rear cover, and pulled the shaft back into position.  I don't recommend it, but it did work OK.    Removing the rear cover is better, perhaps.
 
The fix is to drill the center of that shaft at the back of the transmission, thread the hole, and add a large flat washer, larger than the shaft diameter, and use Loctite BLUE or RED on the STEEL screw you will add to hold the washer to that shaft.   That prevents the shaft from moving inwards.  Use some sort of goop sealant between the washer and shaft/case, to stop any possible oil leakage. This mod has often been done BEFORE there is any or only slight movement, withOUT removing the rear cover.    If you have the gearbox rear cover off when doing this modification, then re-assemble the rear cover by first cleaning the shaft and cover mating areas, and then applying a drop or two of Loctite to the shaft and cover as you assemble the rear cover (heated, of course).   Some other photos of this job; and, a bit more information,
will be found in an article by Matt Parkhouse, in the August 2008 BMW Owners News, on page 38+.

BEARINGS:

4 speed transmission:
   Output shaft, drive (rear) end, type 6204C3, 20 x 47 x 14 mm.  This WAS part number 07-11-9-981-219,
         and that changed to   07-10-1-468-880   Other (front) end, type 6403C3, 17 x 62 x 17 mm.  This
         WAS part number 07-11-9-981-505, and that changed to 23-12-1-338-795. 
   Used on both 4 and 5 speed gearboxes.  SEE remarks below for 5 speed transmissions!

   Layshaft (also called the cluster shaft or intermediate shaft), cover (rear) end, type 6203-C3, 17 x 40 x
         12 mm, the part number is 07-11-9-981-214.

   Clutch end double row type 3202 in some books, but is 3203C3 (aka 5203), 17 x 14 x 17.5 mm.  This
         WAS part number  07-11-9-982-409, and that changed to 07-10-1-468-914.

   Input shaft, both (front AND rear) are type 6304C3  20 x 52 x 15 mm.   Part number 23-12-1-232-695.
          Used on both 4 and 5 speed gearboxes.

5 speed transmission:

RE:  the 6403-C3 bearing 'where the circlip is' :  BE SURE to read the note well up this article on this bearing, and the associated photo of it.

   Output shaft, input (front) end, type 6403 (see 4 speed) (see above too!); and the output (rear) is 6304
     (see 4 speed).

   Layshaft (Cluster shaft or intermediate shaft), both are 6304.

   Input shaft:  Uses special bearing #NU204E at the front, and a 6304 at the rear; and note that the
        6304THNC3 bearing is used, part number 23-12-1-233-807

ADDENDUM's:

In 2005, Matt Parkhouse did a series of articles in BMWMOA-ON, on overhauling the 5 speed transmission. GOOD stuff.

Anton's transmission article.  Highly recommended:  http://www.largiader.com/tech/airtrans/
Listing of parts, some photos, including of the shift kit items, ETC.   Site will further your education on the 4 and 5 speed transmissions.  Anton also lists some of the many changes BMW made to these transmissions over the years, what can & cannot substitute & what won't fit, etc.   
 

Here is a website page that tells you how to overhaul your 5 speed transmission yourself.  Whilst there are plenty of tricks, etc., that are not in this article, it SEEMS to be complete enough that YOU may be able to do an overhaul.    This link was given to me, and I have NOT gone over the instructions step by step.  I have also been informed that the nylon roller that someplace in the article is mentioned as being replaceable by a metal one (maybe he means the K bike metal roller??)....has a wrong part, and the part should be 688, not 628, both being 8 x 18 x 5 mm.  http://jhau.maliwi.de/mot/gearbox.htm
        However, see my much earlier information on that....at item #14, above

SI-00-053-88-new-style-seals.pdf is a file on this website that describes the 'new style' Teflon seal used at the engine output and at the transmission output.  READ IT!
 

Preloading the Shift Lever, what it does to make the transmission shift smoothly!

I will try to simplify this....well, somewhat.  Taking a bit of license here too on the explanations.

Lightly preloading the shift lever can and does work on many motorcycles, and it works on the Airheads and the Classic K bikes.   They are not the only vehicles where this works nicely. PART of the good-effect is due to helping to slow the moving parts just a tiny wee amount during the actual shifting attempt, to help line-up the various parts.

For the Airheads, the transmissions, like most motorcycle transmissions, use dogs on the end (sides, internal and/or external) of some gears.  When shifting, you are moving gears out of mesh or trying to put them into mesh at the gear's teeth, but you are also sliding some gears along shafts (typically the gear is sliding on a splined section) and the side of a gear is try to mesh with the side of another gear, by means of those side dogs; sometimes both are male types, sometimes there are male and female dogs (that is, the female ones are depressed inside the end of a gear).   Thus there are two types of meshing going on, one is the normal gear teeth type of meshing, and the other is the sliding dogs.    End dogs are simply square sections on the sides of gears.  When two such dog sections mesh with each other, the gears are, more or less, locked to each other.  A good cutaway of a transmission or a good sketch will show this nicely.

Here is a really good website that shows how these types of transmissions shift, animated, and also very good single photo of a gear dog end, ETC.
Highly recommended:  http://www.gadgetjq.com/transmission.htm

As you try to shift your 4 or 5 speed Airhead transmission, numerous things must happen at the same or approximately the same time. Some parts must speed up, some parts must reduce speed, in order to enable a meshing (of dogs and gear teeth).   Gearbox manufacturer's have engineers that get headaches from trying to get gearboxes to shift smoothly, compounded by the fact that motorcycle transmissions do not have synchronizers.  For motorcycles, all sorts of friction effects from gears and shafts moving, together with oil viscosity, etc., are involved.  This is a DEEP subject, and one that BMW and its transmission designer-maker (Getrag) have faced, with some strange results now and then ...like adding friction producing O-rings on shafts.   BMW transmissions, properly put together, shift nicely, but other things can work against smooth shifting....AT TIMES.  One of those things is the heavy early model flywheels, as the engine does not like to slow down as quickly between shifts, as does the later lighter clutch/flywheel models (1981+ with the new-style clutch carrier).  Even the 1981+ lighter clutch carrier assembly is heavy and has a lot of INTERTIA, compared to most multi-plate type clutches used on, for example, Japanese and British (and American) motorcycles.  Thus, slowing the engine for shifting takes a small amount of extra time, but that is very noticeable to YOU, the rider.     A lot of things must happen in a tiny fraction of a second.

Aside note:  in SOME types of racing, particularly drag racing, decades ago, fast shifts were considered a must.  Methods included removing some of the dogs, increasing clearances where the dogs meshed.....and an "momentary ignition cutout switch" was installed and was enough to allow the heavily loaded gears to DEmesh and remesh (constant foot pressure on gearshift lever). In fact, many a drag bike rider did not use a clutch!....the shift lever was heavily preloaded, the switch literally "hit" and the bike shifted.

You CAN up-shift your Airhead between gears by preloading the shift lever and using the throttle....and no clutch.......but I recommend NOT doing this.  If you practice, it is not hard on the transmission once you learn to do it correctly.

If the gears/dogs do not line up rather closely at the exact instant you want to shift, then it can take some additional rider effort to get the parts to mesh quickly.  Normally, coming to a stop sign, Airhead riders raise the rpm and shift downwards to first gear BEFORE quite coming to a complete stop.  Otherwise, you may have to slightly engage the clutch or double clutch, play with rpm, etc. when you want to shift from Neutral to first gear when starting off.    In shifting downwards it is normal to blip the throttle slightly.   Experienced riders can often shift downwards without the loud clunk often heard from Airhead transmissions.  Upward shifting is usually easier, although the 1st-2nd transition is a bit longer and longer-taking.   BMW transmissions tend to take a lot of miles to fully break-in.   The transmissions can also shift differently, give a different 'feel' if the oil is changed to a different viscosity grade, and, occasionally, just the brand/model of oil has made a difference.   A Dow additive, in a weak amount, is sometimes tried for balky shifting....sometimes it helps.  


Note: it is entirely possible for two gears to try to mesh but the teeth (or dogs) are directly opposite each other, that is, it is square end of one tooth or dog against same on the other.  Unless you can get one of the two gears to move SLIGHTLY, you will not get meshing.   It is not unusual for this to happen; and it can happen at the side dogs teeth, or the outer teeth of the gears.

Side note:   On kickstarter Airhead models, particularly early ones, if you push the kickstarter down and it seems locked (unless you use a LOT of foot pressure), I suggest you pull-in the clutch lever at the bars, and move the kickstarter lever downwards a bit (easy) and then release the clutch.  That allows proper meshing; due to how the gear setup is.  It also greatly reduces the strain on the parts from otherwise very excessive foot pressure on the kickstarter lever........all of which is torque-multiplied by the mechanical leverage, and applied to, perhaps, not overly strong innards.

Bottom line:  Pre-loading helps improve the friction, etc., to allow closer rpm matching of the internal parts, and thereby easier shifting.
 

DYI (Doing It Yourself) (part 1)

****If you insist on overhauling your own 5 speed transmission, please understand that there are a lot of little things to know.  However, many HAVE done an overhaul successfully; and whilst the overhaul might not be up to 'Guru' standards, it can be perfectly adequate.    I have greatly resisted pressure to do step by step transmission overhaul articles, because of all the little details, which vary by year of transmission.  On the other hand, I do wish to have something for you to look at.  So, I offer the following URL:   http://jhau.maliwi.de/mot/gearbox.htm

READ this entire article of mine all the way through, then go back and read that just-above URL.

After having read my long article you are presently reading, studied it, and are still thinking about doing it yourself, then, go back and read the just above linked-site; and this time click on all the various links, particularly at the bottom, then decide if you want to proceed. That article does not have all the finer details, many of which are in MY article, above.

Bottom line:  If you are the methodical and careful type of person, YOU PROBABLY CAN overhaul your own gearbox (and, if you need some press or machining work, that is easily farmed-out).

Tools and video for transmission work, and other work used to be available from Ed Korn   who previously did business as Cycleworks, Inc. in town of Oregon, WI.  Ed did some machine work, had LOTS of tools (and some parts) for everything from the Isetta cars, through the /2 era, up until the Airheads stopped production in  the late 90's. He had a rather extensive line of tools, some VERY cleverly designed, and he had instructions, videos, all sorts of stuff.    Doing a run-through of his website was informative to many folks.  Ed sold the business to Cycle Works LLC, located at
5805 Haskins Street, Shawnee, KS, 66216   (913) 871-6740.  Contact the new owner at:  Dan@cycleworks.net ((NOT .com!!)).
The URL address is still    www.cycleworks.net   .............Dan will likely be very helpful.
 

DYI (Doing It Yourself) (part 2)
This section was posted by Tom Cutter to the Airheads LIST on June 10th, 2012; in response to some questions, and I leave THEM intact too.
 I have NOT edited all this, except for paragraph separations added and a couple of miss-spellings.
My purpose in putting this posting here is to give you a feel for what is done.
 

""When I do a gearbox job, I heat the box in the oven to 225F, then turn the shafts by hand, shift the gears up and down, and push-pull the input and output shafts to check for grossly excessive free-play. Then I strip the whole gearbox down to the last component, throw everything into the parts washer, turn on the air agitator and go pull the new parts, write the part numbers on the customer bill and on the reorder list, then come back to the clean parts. I rinse the parts, polish the shifter shaft, wire wheel the gasket surfaces, and lay it all on the long table for individual component inspection. That is the most time-consuming part, because each part gets inspected, compared and evaluated for either the "SAVE" or "REPLACE" pile.

I replace all the seals and bearings, so I don't waste any time inspecting those. I clean and sort the shims, and put them into my shim assortment based on size. A this point, all inspection decisions are made, all replacement parts are accumulated and laid out in assembly order, and I'm ready for clean final assembly.

Reassembly takes a very short time, usually less than10 minutes. I first install all the new seals, then stick the gearbox housing in the oven at 275F for 25 minutes. That gives me enough time to replace all the bearings on the shafts, assemble the output shaft and install the little end input seal in the 81-on models. By that time the housing is fully heated, I pull it from the oven, immediately install in this order: Input roller bearing outer race with cage, oil baffles in the cluster and output shaft recesses, the 3-4 shift fork on the stub shaft. Then I engage the cluster gear onto the shift fork, swing it out of the way and lower the output shaft into place. Swing the cluster shaft into engagement with the output shaft and drop both into the bearing bores. Then I engage the two shift forks onto the output shaft, lubricate and slip the shift shaft into the forks and the housing bore.

The shift camplate assembly goes in next. I use my gloved hand to reach down and lift each shift fork into engagement to the selector plates. While the case is still piping hot, the selector assembly retaining bolts are installed from the outside of the case and torqued.

Now it sits to cool down to room temperature. While the parts are cooling off, I complete the repair order paperwork, call or email the client, get a coffee then pull out the measuring tool set for shimming. I sit down to the now-cool gearbox and take all of the shaft and bearing bore measurements, which I record on a small pad of graph paper. (I could use a custom-made form for shimming calculations, but I've been using these little pads for 40 years, and the collection of many hundreds of calculation sheets is kind of satisfying.) This is another "look and think" time. I expect the clearances to measure within a very narrow range. If they do not, I know that there is an anomaly of some kind, possibly a stuck shaft, and assembly error or a flawed replacement part.

Once I am satisfied that all of my measurements are correct, I select the correct shims to get my favorite clearance, and I stick the shims into the cover bores with a dollop of grease. I clean the output shaft taper with some tri-chlor solvent, visually inspect the inside of the gearbox and say good-bye to all those gears, confident that I will never see them again. I set the cover in place on the top of the shafts, and heat using two MAPP-gas torches unto the cover falls into place, assisted with a couple light taps to align everything. Before the cover cools, I install and tighten the nine cover bolts, turn the gearbox over and give a couple light taps on the exposed input shaft. (That seats the bearing in the hot cover better.)

When the cover has fully cooled off, I install the neutral switch and test it for continuity, I run the shifter through all the gears, install the drain plug with a new gasket and the fill plug is lightly screwed on with a large bold tag that says "ADD OIL!" along with recommendations for oil type and quantity for the particular gearbox. I install and grease the clutch throwout bearing and piston assembly, the clutch pushrod and actuating lever assembly. Everything gets wrapped in a clean plastic bag, swaddled in protective bubble wrap and returned to the customer's shipping container to await completion of payment arrangements prior to shipment, or for customer pickup."


<<I'm wondering if maybe you, Oak, and 'Snowbum think we shouldn't have gone to 'Cuda's transmission school, but it was really fantastic.>>

"I have no idea what Oak or Snowbum think. I know that I think it is great that Joe and other guys like him are dedicated to accumulating this knowledge, sharing it and carrying it into the future. That is the only way the knowledge base will continue to grow and survive. It is inevitable that some mistakes in information transmittal will occur, but that does not invalidate the process. Learning is a living, breathing process."

<<I learned so much. I also learned that like you said, this kind of work is WAY over my pay grade. >>

"And it is those guys who learn that who will seek out the assistance of paid professionals. There are plenty of those to go around."

Snowbum snipped some things here, as it is not pertinent.

Tom closed with this:
"(One thing I ask of all of you: This post is not intended as an instruction set on transmission rebuilding. Please don't post questions (on list or privately) asking for explanations of each specific detail. I simply do not have time for it. )"
 

The following are all known by me and recommended by me for ANY type of Airhead work:

"Oak" (Oak Okleshen).  I can recommend OAK for ANY type of airhead work. Real craftsmanship, and very knowledgeable.  Perhaps THE most knowledgeable and METICULOUS craftsman on Airheads in the USA.   Official Friend of the Marque.  MY mentor!  Be SURE to ask about how long your job may take.....he may be backlogged or no longer working.
22637 S. Ridgeway
Richton Park, IL   60471
askoak@aol.com

Anton Largiader in Charlottesville, Virginia   anton@largiader.com  Virginia Motorrad
www.largiader.com   Take a look at:  http://www.largiader.com/tech/airtrans/
  That article has some photos in it that you may find very useful in understanding the 4 and 5 speed transmissions in a few areas.   Anton has two articles to look at, not overly clearly shown as two different links, at least not in MY browser:  Look on the left side, and find 'transmission, clutch, final drive'.   Put your mouse pointer over "transmission" in  "Airhead transmissions and circlip problem".  The word 'transmission' will be seen to be a link to his article. Click on the word.
When finished looking at that article, go back to Anton's HomePage and this time put the mouse pointer over "circlip problem" and click, for the other article.

Ted Porter's Beemer Shop, on the West Coast.   Ted has a deservedly solid reputation, and was with BMW for a long time.  Very knowledgeable. Quality work.  His shop has a good reputation.   
www.beemershop.com
34 Janis Way
Ste E
Scotts Valley, California  
(831) 438-1100

Bob Clement.  
Bob's Motorwerks
often just referred to as  BMW-Montana; Bob has been working with Airheads for decades.
91 Blanchard Butte Rd.
Roberts, MT  59070
406-445-2044
bmwmontana@aol.com

Tom Cutter's Rubber Chicken Racing Garage.
Tom has been working on BMW bikes for decades. 
1360 Colony Way
Yardley, PA 19067
shop:  215-321-7944
cell: 215-206-9787
tpcutter2@aol.com

Matt Parkhouse in Colorado Springs   hobomatt@aol.com or bmwhobo@gmail.com
Matt Parkhouse may have some excellent used transmissions too. 

           The following are known to me only by hearsay:

Charlie Johnson
BMW Motorcycle Service
(all models)
18145 Hummingbird Road
Wayzata, Minnesota
(952) 449-0357

Wuma, Inc (Guenther Wuest)   
wuest@blueriver.net
6891 W. US Hy 150
Fredericksburg, IN  47120
812-472-3739
He ALSO does conversions to the 5 speed transmissions;  information on this conversion will be found in the SidecarURL.htm page on this
website.  Motorren Israel also does different type of reverse gear conversions.

Motor Works, Inc.
1490 Island Ave.
San Diego, CA  92101
619-233-8875   
motowrk@adnc.com

Brunos, in Canada, has a very good reputation.  No personal knowledge, but I think he is trustworth and competent from what I have heard.

 

Revisions: (transmission ID and VIN updates are not listed)

07/07/2008:  all prior updates incorporated, and, minor editing for clarity; and include groove depth; add
                    hyperlink to Anton's site
08/29/2008:  Re-arrange order of some items, change some emphasis, explain a few things more clearly.
10/01/2008:  Relatively major revisions. Few if any specific technical parts descriptions & details were
                    changed, but wording & emphasis was changed in numerous areas, & information on the
                    more accepted type of circlip caused problems was expanded-upon, primarily as the result
                    of someone contacting me who gave input on things that were not clear enough, or really
                    required better explanations.
12/17/2008:  Add photo and description of 4-speed kickstarter idler gear shaft modification
01/17/2009:  Add reference (2) in the addendum section
08/06/2009:  Some modest updates for clarity, and some URL references  recommendations, particularly
                   for the 1974 year.
08/09/2009:  Updated information in several places, including item #14.
10/10/2009:  Go through entire article, and try to clarify as many details as I could
11/01/2009:  update URL's and recommended repair specialist list
12/27/2010:  minor update for 1995 models.
01/28/2011:  Clean up repetitive stuff...somewhat
02/03/2011:  Add 16
06/13/2011:  Add more links
08/01/2011:  Add section on preloading & smooth shifting and theory behind it; fix Cycleworks information
08/04/2011:  Re-arrange article a bit, edit out some superfluous repetition stuff
10/05/2011:  a bit of additional commentary about the shift pawl springs
12/24/2011:  add two hyperlinks
04/28/2012:  Begin revising article for clarity, particularly the pawl spring & shift kit information, which had
                    been scattered in the article.
06/10/2012:  Add DIY part 2
06/17/2012:  Update information on serial numbers and years, and add comments
07/15/2012:  Change this article from 59 to 59A. Split #9 into 3 sections, add link to a new article of mine
                     which is 59B.
10/15/2012:  Add QR code, add language button, update Google Ad-Sense code
11/15.2012:  Re-arrange article. Clarifications.  Better section divisions.  Make width shorter, so might
                    display better on smaller screens.  Fix many html closing tags.
12/13/2012:  Fix poor choice of wording for the description of the two types of throwout bearings,
                    when installed, etc.  Original wording could be misinterpreted.
01/03/2013:  Add photos of 6404C3 special bearing, late type, and the 5 speed gears/shafts
03/30/2013:  Add a bit more information to Transmission Problems, Checks, and Testing.
05/04/2013:  Add transmission rattling noises section, near top of this article.

© Copyright, 2013, R. Fleischer

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