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The four transmission output flange 10 mm 12 point bolts.
ALSO:  push-starting

For: BMW Motorcycles

Copyright, 2014, R. Fleischer


REFERENCE-1:  June 1983 BMW Factory Service Information bulletin (S.I.)  June 1983  26 005 83 (2078)

 There is a fair amount of
                          information in this article by Scot Marburger, about the output flange bolts, threads, theory about threads.


You WILL have numerous occasions to remove or check the tightness of the 4 bolts that are a 12 point type that fasten the forward end of the Airhead Motorcycle driveshaft universal joint to the transmission output flange. This can be for removal of the transmission for a transmission input spline cleaning and lubrication (but, see next paragraph); or, perhaps for a transmission overhaul, or a new clutch or crankshaft output seal, oil pump seal....or some other reason, including replacing the rubber driveshaft-to-transmission boot, and just plain checking those bolts.  You do NOT want them to loosen!

Removing the U-joint bolts is NOT needed for a simple transmission input shaft cleaning & re-greasing (aka 'clutch splines').  It is a good idea to schedule a checking of the tightness of the four U-joint bolts at regular intervals. For that purpose, never loosen first, just check in the tightening direction.  If even slightly loose, remove, inspect the bolt, and if OK, and they screw in smoothly with fingers, then clean the bolt AND flange threads so they are clean and dry & use one or two drops of Loctite BLUE (#242 or equivalent) and retighten to 35 +- 3 Nm, which is same as 26 +- 2 foot-pounds. You can replace the bolt(s) if you think they could have been stretched, or otherwise damaged, or if you just want to be absolutely sure the bolts are OK.     If you have lock washers, remove them, replace the bolts with the later shorter type, which appear almost the same, but are very slightly shorter. Apply the drop or two of Loctite BLUE onto clean, dry threads!!
Don't bother to take into account that Loctite is a MILD lubricant.   NOTE that Snowbum uses ONE drop of Loctite BLUE.

If you fail to use the shorter bolts after removing the lock-washers, you can damage the area behind the flange.  Details further on in this article.

If the flange was totally loosened & U-joint separated (for a new boot, etc.), then EVENLY tighten in a cross-pattern in stages.  If the flanges were separated, be sure the surfaces are clean & totally undamaged by nicks, particularly at the edges, before reassembly. It is important that the flanges have NO metal, not in the slightest, sticking up anyplace that would prevent the flanges from coming totally, flatly, together. Failure to follow this advice can result in massive damage.

The threads on the bolts and in the flange should be oil-free [clean and dry]. This is per BMW, and I agree.
BMW says to replace these bolts (non-washered type) every time they are removed.  I personally do not always do that; what YOU DO, is up to YOU.   See Scot's article, link at the top of this article.
   What I DO is to screw them in with fingers, and if they are smooth ((I assume they have not been stretched ..especially just under the head, visual 10X eye-loupe look-see)), then I usually reuse them, with 1 drop of BLUE #242 Loctite AFTER cleaning the male and female threads with a quick evaporating solvent.  DO NOT spray any solvents into the transmission flange; instead, use a Q-tip or equivalent, moistened with solvent.  Don't use Q-tips that have cores of plastic.  I simply do not want solvents to get on the seal behind the flange.

The bolt COULD have had some stretching just beneath the head, and not in the engaging threads, which would not necessarily be revealed by my screw-in test (less likely, but possible with the 10X loupe look), so this is YOUR decision here as to using new bolts, or not. Certainly new bolts should be the safest.  If you use new OR USED bolts, DO MEASURE THE LENGTH, AND DO INSPECT THEM.

When just checking at regular intervals for tightness, UNscrewing first to check and then re-torqueing for tightness is OK but not a good breaks any existing Loctite bond, and just adds to wear; and you are then re-torquing on used hardened Loctite, which changes things somewhat.  I look at the bolts, threads, and at the sharp point of shank to head, beneath the bolt head, using my 10X eye loupe....I look for almost microscopic cracks or other distress.
   I have NEVER had a reused set of these bolts break....nor loosen....that I personally installed. YMMV!!


The earliest bolts for the /2 and the Airheads were used with split type lock-washers, and were not torqued as tightly, per specification, as the later non-lock-washered ones (20-22 footpounds was generally used early-on, but there were even lower specifications quite early-on), so these early lesser-torqued ones would not have stretched, unless considerably over-tightened. If you greatly over-torqued the bolts...considerably beyond 30 ftlbs I think... you might even break them or damage the flange threads area, besides stretching threads. 

>>>  I will NOT use the early style 'long' U-joint bolts with any type of washer, and especially not split washers.  You WILL have serious problems if even just one washer cracks or breaks or has spread slightly during tightening...these things allow bolt loosening & has been known to happen well after assembly.  Some have used waverly lockwashers, I think I should call them anti-loosen or vibration washers, but I do not use them, not ever.    I use ONLY the later, slightly shorter bolts, withOUT lockwashers.  I use blue Loctite on already cleaned and dried threads.  

It is acceptable to tighten those bolts by using a hand wrench and not a torque wrench, this is for Field Use, that is, you are on a ride/tour, and have a reason to tighten the bolts.   So, how do you tighten these bolts if no torque wrench?:  Use a 12 point box wrench wrench that FITS the cramped area AND is about 6" long, and use "a good grunt" on the bolts using that wrench.  You probably already have that particular wrench,
it is somewhat thin-looking but not fragile.  Here is the description in my tools article,
71-11-1-237-847   10 x 12 box end wrench, which replaces 71-11-1-230-579   

To measure the bolts length, to be sure you have the correct ones:  measure the distance from the underside of the head to the end of the bolt.

The official book value on the old bolts, 26-11-1-230-414 were 14.5 mm (actually measured by me at 14.4 mm or 0.568").  These were the longer ones that were used with the troublesome lock-washer 07-11-9-930-840.   NEVER EVER reuse these lock-washers, unless a dire emergency.  If you absolutely have to reuse these old longer bolts, install new lock-washers, even if from the hardware store.

The new bolts, that began being used in 1983, are 13 mm, and part number 26-11-1-242-297.   That means a 1.5 mm (0.059") difference, officially. 
BUT....In practice, the bolts will ACTUALLY MEASURE almost exactly 1/2 inch; or, for the metric folks, 12.7 mm and a difference of  1.7 mm. ... these very slightly shorter bolts MUST be used if the lock washer is NOT used, to avoid seal damage.

Could you shorten the original bolts and use them withOUT the split lockwasher:  YES, but you must NOT heat the bolt much during the grinding/shortening.  The bolts are very tough hardened metal, NOT change their heat treatment by getting them quite hot. So, if shortening the longer ones, do a SMALL bit of grinding at a time, allowing cooling between grinding attempts.  If the temperature of the bolt during grinding is hotter than you can comfortably hold in your bare hands, it is likely too hot.  Do NOT leave messy sharp threads, etc., at the end.

BMW says that at 29 ft lbs, the bolt stretches. I do NOT believe that, and I think it takes a fair amount more.   A bit more nerdy explanation is that BMW could be correct, but I believe that at 29 footpounds the bolt stretch is NOT PERMANENT.

Some manuals and literature have the torque at 27.5-31 foot-pounds for the 1977-80 models, and 29.0 for the 1981 models. Note that these were before the 29 specified by BMW in 1983. The last of the Airheads, owners manual, says 35 +- 3 Nm, which is 26 +- 2 foot-pounds, and I certainly can agree with that.  

When using a torque wrench, using an extension adapter straight-out, the adaptor extends the working length of the torque wrench, increasing the actual torque from that indicated by the wrench setting.   The torque wrench setting needs to be reduced to compensate.  Information is on this site on how to adjust for that factor, link at end of this article. Snowbum uses it STRAIGHT OUT.

 When using a torque wrench with an added wrench (or adapter) at 90 to the torque wrench barrel, no correction is needed, and you can just use the actual desired torque setting on the torque wrench.

Do NOT use RED Loctite, it requires a LOT of heat to loosen and remove a bolt installed with RED Loctite, and if you do use RED Loctite, it is my belief that you MUST discard the bolts and use new ones, each and every time you have used RED Loctite. 

Using any Loctite (Snowbum uses ONE drop of BLUE, a medium holding Loctite) gives some added insurance, while not acting hardly much as a lubricant, which increases effect torque.

How much measured torque does Snowbum use on these bolt heads?   I set my wrench for 26 footpounds at the bolt head.  
Note what I said earlier:  "The last of the Airheads, owners manual, says 35 +- 3 Nm, which is 26 +- 2 foot-pounds, and I certainly can agree with that."    Torque wrenches are not often re-calibrated, or calibration checked.  Snowbum DOES check his.  Because of variances I have seen, do NOT use a setting that gives over 29 footpounds AT THE BOLT.

HISTORY of the bolts and washers........and push starting.........and some discussion:

The transmission output flange is a TAPER FIT to the transmission output shaft.

This is why bump starting is a poor idea, although acceptable, but NOT IN FIRST GEAR, and probably not unless careful, in second.

Forces on the flange & bolts are substantial, especially during shifting gears or sudden braking.  The ACTUAL transmission of power is, via the taper, the flange contacting the FLAT SURFACES.... but those are pressured together BY THE BOLTS under discussion...which do carry a shear loading.  BMW has had some problems with these bolts backing-out (unscrewing, becoming loose, use whatever words you like) over the years.

In 1983, BMW eliminated the lockwasher, as many had broken (back even to the /2 days!).  The result of an even slightly loose bolt was that the bolt would continue loosen....and soon all 4 were loose, or, even before, the U-joint and transmission flange parted company.  When the flange and U-joint parted company, the rear of the transmission could be torn off.  This had happened early-on to a R60 bike; which is how I purchased it so cheaply!

The original longer bolts, with the split lock-washers, at one time had a tightening specification of 17-19 ftlbs, and that was likely fairly adequate then, due to power being transmitted.

I have been informed a few times by various owners that BMW dealership parts departments have stated that BMW has either gone back to the washer design, or at least some dealers are/were selling the original bolts and lock-washers.  DUE TO A MIXUP BY BMW, ANY INFORMATION ABOUT A REVERSAL TO THE OLD STYLE BOLTS AND LOCK-WASHERS IS NOT TRUE.  Only the new shorter bolts are to be used, withOUT washers. 

I have no intention of mentioning my sources for this information/
Some time after the bulletin came out (see very beginning of this article for bulletin number), BMW discovered that they had a large amount of the old longer bolts & washers in stock, & some dipshit decided that it would be a good idea to distribute a "Parts Bulletin" recommending that they be used for the pre-1980 was a Parts Department guy who wrote it; he vanished from BMW shortly thereafter,, because the warranty claims started coming in again.

Adaptor tool for your torque wrench, for your shop, not necessarily for the bike tool tray:
For years I used a 10 mm 12 point BMW tool/adapter an old number for it was 88-88-6-002-560, manufactured by Hans Schubert in Germany for BMW, and there may have been other makers at various times. Mine was sold with marking as "00-2-560" for a part number.  MAYBE this tool is still available, but the price in my 1995 BMW book was $54.58 retail and $32.75 dealer cost. That is pretty pricey, even back then.  It is true that one can use a double ended BOX wrench, and then an Allen adapter to your square drive on the torque wrench.   I have seen combo box wrenches with 10 mm on one end and 8, 9, 11, 12 mm on the other.    The original BMW 12 point box end wrench which some use for this purpose is OK.  You must either use that wrench with a goodly grunt; or, use it with a torque wrench, and calculate the reduced torque wrench setting (unless the usage is at 90).  

The actual real BMW tool mentioned above (that I owned), was simply a square female drive with a slot.  The actual tool is simple, and PICTURES and descriptive information is here: in the  It works very nicely with a torque wrench.  There are now at least two sources for a small version, one-piece, laser-cut, that is quite neat-O.  See that tools article, it has photos of the BMW tool and aftermarket tool.  I am FINE with you making your tool, or, purchasing the aftermarket too.  I think BMW's price for the tool, if even available, would be outrageous.

For those that do not fully understand torque wrenches and adaptors at various angles, please DO NOT use your 'adaptor' at anything except almost perfectly straight out (that means parallel...IN LINE WITH THE TUBE OF THE TORQUE WRENCH) to the clicker torque wrench.  That way, no angle is involved to mess up the calculations.  That makes calculations easy, and no leverage problems or interferences.  Do the calculations and reduce the torque wrench setting appropriately, to compensate for the extension. STORE that clicker wrench with its setting close to zero (standard advice). 

NOTE, however, that if the adapter is at 90 degrees, no calculations are needed.

Here is how to do the calculations for YOUR torque wrench:


IF YOU HAVE TO PUSH-START, IT IS BEST DONE IN 3rd GEAR, NOT FIRST GEAR!  While one reason is that 1st or 2nd might cause the rear wheel to slip and not transmit enough force to rotate the engine is also easier on the flange taper to not use sudden very high shock forces when suddenly engaging the clutch.  You do NOT want the taper fit to slip....that usually damages it.
If you are on dry pavement, and are careful, you can use 2nd gear.

04/04/2003:  revise strictly for clarity
05/10/2003:  add some items previously in hints.htm section of website, and revise entire article.
01/14/2004:  Clarifications about the -560 wrench, use of the torque wrench, and hyperlink to the tools.htm
02/15/2005:  a few more clarifications
01/30/2008:  Clarify that 29 footpounds is the maximum, and that 20-22 is likely OK with Loctite blue;
                    minor other changes (grammar)
03/08/2008:  Add exact measured bolts information, in bold and color; do some minor clarifying
06/17/2011:  Clean up article, add hyperlink, etc.
09/26/2012:  Add QR code, language button, update Google code
09/27/2012:  Clean up for clarity regarding the new bolts, torques, push-starting.
03/29/2014:  Clean up possible confusing information on torque wrench use/settings.  Two more edits,
                    one later in the morning, and one in the evening.
08/16/2015:  Clean up a bit, and fix excessive verboseness in two places, and set down latest specs for torque.
08/22/2015:  Fix error in bolt part number, plus some minor clarifications and cleanup.

Copyright, 2014, R. Fleischer

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