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The BMW Airhead Oiling System
Sketches, descriptions; caps; thermostats;
oil breather ...ETC!

Copyright, 2014, R. Fleischer

This article is expected to be used together with article 50A.  I suggest you find the time to read all articles from #49 through #51D.


First sketch is of an EARLY Airhead oiling system (through approximately 1979).  It is from a BMW Factory Service manual.  All  Airheads from /5 have a similar system. 


EARLY system, through about 1979:

Sketch item item #


           1 Oil strainer pickup in engine oil sump
           2 Pipe from above strainer to oil pump
           3 Oil pump, Eaton-type
           4 High pressure oil pump output, note the two engine plugs in the passageway.  This output goes to the oil filter canister.  Note that this oil goes to the OUTside of the oil filter.
           5 Oil filter canister and oil filter

This item
should be
using a
at every
oil filter

Oil pressure bypass valve located at the innermost end of the oil filter metal canister.  The spring-loaded ball-check valve device allows unfiltered oil to pass into the engine from the pump & canister if the filter oil flow is blocked.  This ball-check valve has very rarely come loose; you may find parts in the canister area.  More often, but still quite rarely, the spring has broken; bits of it gets into the oiling system...bad news, as considerable damage is possible.  To replace the valve, clean the threads with a good evaporating spray solvent; then apply BLUE (medium strength) Loctite or equivalent, in a SMALL AMOUNT to the threads.   DO NOT get any on the ball & its seat.  There is no specification on how deep to install the slotted part, so estimate NOT screw it in too far, you will change pressure characteristics.   
           7 Outlet of the oil filter canister, leading to the camshaft
           8 2.5 mm diameter bore passageway in camshaft flange
           9 Oil from camshaft flange to engine front main bearing cap
          10 Passageway with outlet (#16) to REAR main bearing and to the pressure release valve (#11)  
          11 Pressure release valve, opens above 5 BARS (75 psi).  Excess output lubes the chain & sprockets area.   Oil FROM this valve can be small to non-existent if the oil is hot and engine rpm is at idle.
          12 Front big end bearing shell, has outlets to #13 & #14
          13 Passageway to RIGHT cylinder head via the TWO top studs
          14 Passageway to LEFT cylinder head via the TWO top studs
          15 Bore IN THE CRANKSHAFT delivers oil to the LEFT big end.
          16 Passageway to the oil pressure switch AND rear main bearing
          17 Bore IN THE CRANKSHAFT delivers oil to the RIGHT big end
          18 Oil pressure switch.  
          19 Oil return from cylinder head via pushrod tubes & lubes camshaft lobes and lifters along the way
This item should
be checked for
chipping, etc.,
whenever you
are in the area
or suspect
excessive oil
Crankcase breather, disc style, later was a reed valve here.  The disc is ~1/16" thick, 27 mm diameter 0.22" (#2 drill) hole.  Be sure to read the information on the breather disc, later in this article!
Source for newly made old-style Airhead breather discs ...$5.00 for each disc and $4.00 for each O-rings; which includes USPS.
Dave Thompson
           21 Oil condensing chamber
           22 1.5 mm diameter hole in bottom of #21, not found on /5 models.  Returns condensed oil to the crankcase.  See photo below on engine cutaway photo
Cover for oil canister.  Type shown is for the oil cooler model as shown; which incorporates cooler feed line #24 and return line #25.  GS models will have a NON-thermostat cover.
Thermostat, on
cooler models,
when equipped
with the
type of cover
The thermostat seldom fails.  They have been known, RARELY, to stick.  This does not have any real effect on engine oil flow.  The thermostat is NOT simply an on-off valve.  The valve inside it determines what percentage of oil is routed to the cooler.   The thermostat is specified to begin to open at 80C (176F) and be fully open at 110C (230F).  GS models without the thermostat use a sized hole to control the flow to the cooler, versus temperature, due to oil viscosity change with temperature.  This method seems adequate, although using a lot of rpm with the engine oil itself, at startup, AND being at very low temperatures, MIGHT be hard on the cooler soldered/brazed seams.  The GS cooler is also supposed to be COVERED in really cold weather, to avoid OVERcooling the oil.

Oil is also returned to the sump by the amount escaping from the various bearing clearances.

Later system (from approximately 1980):

In '1980', BMW changed the engine casting oiling system internally.  BMW re-routed the oil so it now went directly from the filter to the front crankshaft bearing. On the way to the front crankshaft bearings the oil went to the camshaft flange, via an angled path.  This change was incorporated at various VIN numbers ...that is, it was phased into production.  Because of that phase in, certain swapping of engine castings/components might require some modification (front main bearing cap, for instance); the information is in a bulletin from BMW, which I have on fiche; and, for which I have on file in this website: modifications-2031.pdf.   That document has other information, such as modifications to the rocker arm shafts oil passage, & cylinder head torque.  DISREGARD the factory cylinder head figures, use only 25 foot-pounds for ALL cylinders, all years, all models; as per the note I placed on that document.  This phase-in, as far as I know, did not occur exactly simultaneously with the change to a simplex (single roller) chain; nor to the change to/from the canister points ignition.  BMW marked the crankcase for identification when those oiling system changes were phased-in.   The CLUTCH side of the engine casting has a triangle with an N in the center of it ...this is cast-into the crankcase there.   This mark is the standard Alcan mark.

The phase in occurred as follows, per BMW;
it is POSSIBLE these numbers are NOT necessarily absolute:

R65, phased in at 6381867
R80/7, phased in at 6126171
R100T, phased in at 6170415
R100S, phased in at 6165103
R100RS, phased in at 6185422
R100RT, phased in at 6196045 (6196044 was reported by its owner, with a photograph as proof, posted January 20, 2014, to the Airheads LIST, to have the triangle with N mark).  That particular bike serial number shows it to be produced in November of 1979 ...thus it is a 1980 model year bike.

There is a oil pressure bypass valve located at the innermost end of the oil filter metal canister; same as prior models; see item #6 description and cautions, in the table, several paragraphs above. 

NOTE:  In the below sketch, the sketch-maker goofed, and is still showing the DISC breather ...which was replaced with the reed valve type by BMW.  For you folks with the older disc type, that type works OK, if in good condition, although they can get noisy.  Repeat of information in my chart:  Crankcase breather, disc style, later was a reed valve here.  The disc is ~1/16" thick, 27 mm diameter 0.22" (#2 drill) hole.  Source for newly made old-style Airhead breather discs, $5 each, $4 for O-rings, including USPS shipping:  Dave Thompson,

MORE on that breather disc, etc., later in this article.

Internal cap models (and not), coolers (and not), thermostats (and not), oil flow, how the oil moves at the canister area, ETC:

The single bolt internal cap models are not equipped with a cooler.   They also do not have the later type of outer cover.  A cooler can be installed, but that involves removing and replacing the central pipe AND the canister.  The oil flow inside the single bolt canister is rather similar to the later or thermostat or GS models.  The canister, closed & sealed by the single bolt cap cover, obtains its oil from the oil pump, which is applied, like all the BMW Airhead filter chambers, to the outside of the filter.  For the single bolt internal cap cover models, the central pipe is much shorter, therefore the oil flow passing through the filter goes more directly into the shorter central pipe.  

The thermostat, on cooler models so equipped, seldom fails.  They have been known, QUITE RARELY, to stick.  This does not have any bad effect on engine oil flow.  The thermostat is NOT simply an on-off valve for the oiling system.  The valve inside the thermostat determines what percentage of oil IS ROUTED TO THE COOLER.    The thermostat is specified to begin opening at 80C (176F) and be fully open at 110C (230F).

GS models do not use a thermostatic plate. They use a sized hole to control the oil flow to the cooler.  While not sophisticated, in practice it is adequate, although using a lot of rpm with very cold engine oil at startup, MIGHT be hard on the cooler soldered or brazed seams.  The GS cooler is supposed to be COVERED in really cold weather, to avoid OVERcooling the oil.  

If it is still not clear to you exactly what the path of the oil is in the canister and cooler:
The high pressure oil output from the oil pump goes directly to the oil canister chamber, such that it is applied to the OUTSIDE of the filter.  The oil passes through the filter and then to the outer filter end, where sort-of slots in the metal filter cover allow oil to pass to the right.  The oil goes into the outer cover via a hole that is offset from the center hole.  The filter right end is semi-sealed to the cover by the square-sectioned O-ring.  In the thermostat models, that outer cover hole is 8 mm in diameter.  On the GS models, early versions had the hole at 2 mm, but they should be drilled out, if you have that one, to 4 mm (5/32").  The oil flows into the cover plate and immediately out of the plate into the central pipe, which is a light fit into the cover.  Thus, it is important, if checking the central pipe for tightness, that you do not make burrs on the pipe end.  The pipe must stick into the cover central hole.  The pipe normally sticks into that cover central hole about 3 mm.  The central pipe is the route for the oil to get to the engine oiling passageways.

When the thermostat gets to about 176F, it starts to open the passageways allowing SOME diversion of the oil that normally went from outer hole to inner hole ...the diverted oil goes to the oil cooler.  Diversion happens constantly (no thermostat) in the GS models.

Because of the way the system was first designed (/5 era), and later modified by BMW, it is CRITICAL that the oil canister not leak oil back to the oil pan.  That sealing is done by the cap cover on the early models, and by the large white rubber O-ring, on later models.


The oil pan has been changed several times over the years.  First to deepen it half an inch to give additional room for crankcase pressures/blowby and again for even more capacity.  The drain plug was moved to the rear.  A baffle was installed to better keep the oil in contact with the oil pickup during hard deceleration.  All oil pans are an interchangeable fit, as they all have 14 bolts in the same place.  However, any center-stand crossbar must be considered. BMW changed that, too.   The later pans are better than earlier pans.  There is an article on this website that has information on dipsticks and fitting the pan gasket, to which you must NOT apply sealant.  The cork gaskets of the old days are NLA.    Do NOT over-torque the bolts ...I don't go much over about 6 footpounds (72 inchpounds), and actually do it by FEEL, and tighten them quite evenly all around, IN TORQUE STAGES, using a cross-pattern.   Recheck tightness after a few rides.  Do NOT over-tighten!  Whenever the pan is off, check the bolts for the oil strainer pickup.  Check the pickup parts, especially the earlier pickup connection area, for cracking.  

In 1977 BMW enlarged the oil pan capacity.   In 1981 BMW made changes to the oil pan AND the pickup.  There had been a few instances of the all-steel pickup tube assembly cracking, which often meant sucking air, not oil.  BMW went to a dual-piece pickup & aluminum adapter with 2 bolts.  Early models used a composite adapter, which should be changed to the aluminum type, because the bolts tended to loosen over long periods of time, again allowing sucking of air. Use Loctite BLUE on clean & dried threads when assembling (& fresh gaskets).   The enlarged (again!) pan now had a baffle in it.  The purpose of the baffle was to eliminate oil starvation at the oil pickup screen during very hard braking.   The parts are available and are a popular conversion.  Anton Largiader's website,, has more information.  Anton also has a lot of information on the dipsticks, various stock and aftermarket pans, etc.

Breather system:  

The sketches are not all that clear about the layout of the breather system.  The factory sketches near the beginning of this article both show the original round-disc type, but do show, and NOT clearly, the oil condensation return hole #22 as incorporated later.  The PDF sketch can be expanded to show more detail, but the information in the below section should be helpful to you.

The breather system varies from the early models to the later models, but the basics are the same.  The crankcase breather is located in the starter motor area, in two sections, to the starter's right. Oil and moisture fumes are drawn into one or both carburetors depending on year and model, and burned during combustion.  The two breather system changes that are the most important are the drain-back hole, and the change from the round disc to a reed breather.

Engine Cases FROM 1977 or 1978 had a tiny, 1.5 mm drain-back return bore hole, item 22.  In this area much of the breather vapors condense and are returned to the crankcase.  Item 22 is called the "Return Bore in Settling Chamber" whil the somewhat harder to understand item 21 is the actual settling chamber.  Item 20 is the breather valve.  Shown in the sketches at the top of this article are the earlier spring loaded (and adjustable to two positions for smaller versus larger engines) round disc type.  The disc type tends to get chipped edges and also sometimes makes noises, often called or described as a Turkey Gobbler, or Honking. The noise may come from the original main seal however.  A new disc can be made up from something like printed circuit board material, if one wanted to.  The later reed type can be installed in earlier disc type engines.  There is really nothing wrong with using the disc type, although they sometimes makes noise.  I actually prefer the earlier disc types.  If your early disc is making objectionable noises, and the disc is in good condition, and mounted with the correct clip position, you can usually eliminate the noise by drilling, carefully, ONE ONLY 3/64" hole, at approximately the 50% of radius position.  The early model breather looks like a round disc with a small spring and a holding clip on the shaft. There are two grooves in that shaft, the top groove is used for the R50/5, R60/5, R60/6 and R75/6, and the bottom groove is for the R75/5, R90/6 and R90S. The round type of breather valve can be removed and the later style installed.  There is nothing wrong with the operation & design of an early disc type breather valve if the disc is in good condition.  It is arguable if the later reed type works better, but it IS quieter & does NOT seem to ever wear out.  You can make a new round disc from approximately 1/16" thick fiberglass electronics printed circuit board material.   The diameter needs to be 27 mm  (about 1.06"); and the center hole #2 drill bit (about 0.22"). SEE THE NOTE, above in RED, about drilling a hole, if need be, for objectionable noises.

Source for newly made old-style Airhead breather discs:
$5.00 for each disc; which includes USPS.  Dave Thompson

If your motorcycle is making a sort of birdy sound, squeaks, chirps, maybe honks, usually only at idle rpm, it may be the MAIN SEAL that the flywheel goes into.  Remove the old seal and install the latest type.  Detailed information on doing that is in this website in a different article.

For a few months in the 1978 production cycle, the breather system fed both carburetors.  This is, of course, for the clam shell air cleaner models (the rectangular models came with the breather feeding both carburetors). 

The breather system appears to be simple, yet is more complex than one might think, with settling and quieting chamber(s) and using very faint vacuum at the carburetor intake to allow it to function properly.  The breather system also has potential problems. 
If the round disc is at all damaged, especially if cracked or chipped at the edges, it will cause high oil consumption.  The small drain hole added after the earliest models must be clear, not clogged.   This hole is often not seen with a quick glance.  That hole allows condensed oil to return to the crankcase.   If this small hole is not clear, oil consumption will also be high.  DO poke a wire, or?, into that hole during inspecting the breather.   The actual system involves not just this disc valve, or reed valve area, but the outlet area forward and to right of the starter motor.

This is the ~1978 and later engine case, cutaway, showing the drain-back return hole mentioned above.  NO, I do NOT know if anyone has drilled an earlier case for it, but see no reason it should not work.


UPDATING the breather valve to the later reed type (if you want to):

To update the valve you must remove all parts except the post, and then fashion a way to pry-up or pull-up the post.   You can fashion a puller for the valve using socket and screw or make something from braided steel stranded wire, or other means, including a small puller.  HEAT the case area fairly hot around the old round valve stem before trying to pull it out.   You can also purchase a simple tool set to do this job from an aftermarket tool supplier:;
try this URL:

Other methods of removal:  You can remove the post, drill the center, install a bolt and nut and yank it out using Vicegrip pliers, or some other means, with a couple of makeshift pry bars under that pliers. DO NOT nick the gasket surfaces.

The new-style reed valve consists of only three pieces, which include a membrane (reed) and a screw.  Be prepared to install so that the cap fits properly.   Depending on the model, you may find the screw needs to point to the right foot peg, perhaps at about 5:00, tapping the assembly using an appropriate size of socket that clears the top reed stop plate.  Use RED Loctite on the screw threads.   Another way of saying this is that the open end of the reed faces (points-towards) the left handgrip ...about a 10:00-11:00 positioning.    LOOK at the cap, and LOOK at the reed opening.

I suggest that the case be heated when installing a reed valve assembly. FREEZE the new valve; then install correctly oriented and very squarely, into the heated case, and do it quickly.  I use a socket and hammer as a driver.  It helps to have the hole clean, no sharp edge; and to have a smooth new valve...... no sharp edges.

If upgrading to the reed valve on a clam-shell air cleaner housing model Airhead, you may want to upgrade the cap, using cap 11-15-1-335-756; hose 11-15-1-338-215; and gasket 11-15-1-338-431.  The actual breather valve assembly is 11-11-1-335-712.   The early R65 can use these too.   These other parts are only really needed for these bikes of the clamshell variety (and earliest rectangular models???) ...where the venting is directly to the airbox, with no intermediate junction.

NOTE!! ....use of the reed breather should be considered a must if you are racing and using very high RPM ...this is particularly so if you installed the racing camshaft, the so-called "336".


The pressure sensor switch port might have 14.5-29 psi (1-2 Bars) at about 800-1000 rpm, and perhaps 60-74 psi (up to roughly 5 Bars) at 4000+ rpm.   This all varies considerably with temperature of the oil, viscosity of the oil, and condition of the various parts of the engine.  The switch threads are 12 x 1.5 mm.  /5 threads and some /6, were PIPE threads. 

A RARE event, but has been seen now and then, is an engine with the front main bearing having rotated, which cuts off oil to the rocker arms, and lowers oil pressure.  You will usually find a steel pin in the oil pan.  While the main bearing is a press-fit; if the pin, which is supposed to be pressed-in and staked, comes out (big oil pressure is there, helping to push out the pin), then the bearing MIGHT rotate.     This is a SERIOUS event, and usually requires the entire front of the engine to be disassembled.   You can run the engine, and the bearing may not rotate, but you are taking a real chance at serious damage occurring.  If you find such a steel pin in the crankcase, you need to think about the consequences if the bearing rotates and you do not fix the problem.

If you have the flywheel off, be sure to check the oil pump cover.  If it has the phillips screws, I suggest you update to the later cover and later bolts.  The newer cover is also different in its oil seal groove, and sealing is improved.

09/21/2003:  Add information on phase in identification numbers and the Alcan mark.
08/24/2004:  Add switch thread information.
07/05/2005:  Revise for clarity of years and models, etc., the breather information.  
12/21/2006:  Completely revise, with much clearer and very much larger sketch, edit the text for clarity, add a table for the 25 sketch items.
02/03/2008:  Remove engineinternals.htm hyperlinks.
04/28/2008:  Add thermostat information.
05/08/2008:  Minor clarifications and editing AND add the 1980+ oiling sketch.
01/03/2009:  Add information to clarify how oil moves in the canister area.
10/17/2009:  Add photo of settling chamber hole in a cutaway casting view.
11/24/2009:  Add .pdf of the 1980+ system.
03/18/2010:  Straighten out wording on the thermostat function in the CHART section.
02/24/2011:  Combine with the reed valve photo, and edit entire breather system area
03/23/2011:  Remove oil pan information from title.
10/12/2012:  Add QR code, add language button, update Google Ad-Sense code.
02/23/2013:  Clean up article in numerous ways; add more notes here and there, clarify things.
02/27/2013:  Add notes on availability of new discs from Dave Thompson.
10/20/2013:  Removed the following commentary & hyperlinks, as bmwscotter's website is down for non-payment of fees.  This section was in the area so noted just below the photo of the reed valve.   "Here is a step-by-step procedure, and lots more photos and other stuff on the breather system....there are some differences over the years....  For a bit fancier methods:
09/23/2014:  On BMWMOA Forum, was some commentary, regarding his poor health, and move to New York, selling all his airhead items.  That's all I know at the moment. When I find out if his information is posted someplace on the Internet, I will incorporate it.   I later found out that the domain is for sale.
01/21/2014:  Add note regarding serial number for the front main bearing cap area oiling system change PHASE-IN serial number.
03/23/2014:  Could not fix link to the German pdf oiling system, security deleted.
07/14/2014:  Remove note on Dave Thompson having 'O-rings", which does not belong with the breather disc!
07/22/2015:  Add 2031.pdf to the website and modify the information in this article, and to refer to it.
11/10/2015:  Add additional explanation about full-flow and bypass flow.
03/10/2016:  Update A/R, including metacodes, formatting, fonts, layout, etc.
09/27/2016:  Updated metas, scripts, layout; set photos for no wrapping of text.

Copyright, 2014, R. Fleischer

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Last check/edit: Tuesday, September 27, 2016