Snowbum's BMW Motorcycle Repair & Information Website

The thermostat, on models so equipped, almost never totally fails.  They have been known, quite rarely, to stick.  This does not have any bad effect on engine oil flow.  The thermostat can be damaged by using the wrong length and not properly rounded nose for the refilling (air bleed actually) bolt.   The thermostat is not simply an on-off valve.  The thermostat determines what percentage of oil is routed to the cooler.   The thermostat is specified to begin opening at 80°C (176°F) and be fully open at 110°C (230°F).  GS models with their frame being more in the way of the oil canister cover plate do not use the thermostatic type of outer plate, and without the thermostat they use a sized hole to control the oil flow to the cooler.  That, in practice, seems adequate, although using a lot of rpm with very cold engine oil at startup, is likely bad for the cooler's soldered/brazed seams; so see below on the proper hole size.  Starting ANY Airhead engine model with quite cold oil and then using too high an idling RPM, or getting under way with too high an RPM, can cause serious damage to the oil filter element and possibly the cooler.  Resaid: If starting any Airhead engine at 40°F or below (engine temperature), do not use excessive RPM during startup and initial running.  Avoid more than slight throttle blipping. The oil pressure in the oil filter area can be so high as to collapse the oil filter element or damage the cooler. This is a good policy no matter the temperature.

The GS cooler is supposed to be covered in exceptionally cold weather, to avoid overcooling the oil.  Yes, some folks ride in conditions WAY below freezing.  I do suggest that if you are going to ride in temperatures below freezing, that you consider covering the cooler.  You can fashion a cover that stays on.

Exactly what is the path of the oil?
I have factory oil sketches on this website, but here is a very short & simple explanation.  The oil in the oil pan is 'sucked-up' through a very coarse strainer, and then goes to the oil pump.  The high pressure oil output from the oil pump goes to the oil filter canister chamber & is applied to the outside of the oil filter.  In the earliest models, like the /5, the oil goes through the filter, and then into the engine via a short pipe on the inside of the filter element.  The filter canister chamber outer end has a metal cap cover with a large single bolt fitting through the cap.  That style ....and use of the flat plate covering the filter area access hole ...requires a paper gasket.

Later models do not have an internal metal cap cover.  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.  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 smaller O-ring.  In the thermostat models, that outer cover hole is 8 mm in diameter.  The oil flows into the cover plate and immediately out of the plate into the central pipe, which is longer than the /5 no-cooler type, and a light fit into the cover, as the cover is installed during a filter change.  If checking the central pipe for tightness you do not want to make burrs on the pipe end.  The pipe must go into the outer cover central hole.  The pipe 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.   Cooler equipped models have a modified outer cover, to divert oil to the cooler.  If further curious, check the sketch article which likely will clarify and also add valuable details.  Both early and late system sketches are included, with descriptions!  https://bmwmotorcycletech.info/oilsketch.htm

On all Airheads, if the filter should get clogged in some fashion and oil not get into the central pipe properly, the pressure increases in the oil filter chamber.  A spring loaded ball-valve, located at the far inner end of the canister, will open, allowing some oil to flow into the engine.  The ball check valve should be inspected with a flashlight at oil filter changes.  There have been instances of that ball check valve spring disintegrating....which could allow metal to go directly into the sensitive engine areas.  There is little if any filtering, if the ball check valve is not intact and functioning.  Nerdy:  The ball check valve begins to open at approximately 1.5 Bar (about 22 psi), but that is the DIFFERENTIAL pressure...the pressure inside the canister versus the pressure on the canister output.   To explain that a wee bit, the canister has the oil pump output pressure applied, and that means one side of the spring loaded ball also is exposed to that pressure.  In the output of the canister passageway, the other side of the ball is exposed to that.

The ball-check valve has very rarely become loose, and when it does, sometimes you find parts in the canister area.  Somewhat more often, but still quite rarely, the spring has broken, and bits of it gets into the oiling system ...this can be bad, as considerable damage is possible.  If you have to replace the valve or otherwise repair it, clean the male and female threads with a good evaporating spray solvent, and then apply BLUE (medium strength) Loctite or equivalent, in a small amount, to the clean threads.   Do not get any Loctite on the ball and where it seats. Once in place, I suggest you immediately oil the ball/spring, to avoid the possibility of Loctite having gotten to the ball and carrier.  There is no specification on how deep to install the slotted holding part, so you will have to estimate it ...do not screw it in way too far, you will change pressure characteristics.   This caution paragraph is repeated elsewhere's in this website, and you can get a better idea of the oiling system by going to:  https://bmwmotorcycletech.info/oilsketch.htm.
There are two sketches in that article, and some explanations, etc.

When the thermostat begins to heat up, and reaches ~176°F, it starts to open the passageway allowing some diversion of the oil that normally went from the cover plate outer hole to inner hole, and the diverted oil now goes to the oil cooler.

On the GS models with oil cooler (the GS has a non-thermostat outer cover), there is a factory bulletin on the early outer covers.     The bulletin says to inspect the small hole in that cover, and if 2.0 mm, to drill it to 5/32" (that is 4 mm) diameter.  That is the bypass port hole, the function of which is to allow oil to flow, even if the oil is quite thick when cold.  Increasing the hole size reduces the amount of oil passing through the cooler.  Speculation is that with the original smaller hole, some coolers ruptured with starts in quite cold weather from the very high oil pressure when the oil is very cold, probably especially if the throttle was blipped to too high an RPM at that time.   That hole is smaller in diameter than the hole in the thermostatic type covers (which is ~ 8 mm). The GS cover uses that specific internal hole size to always allow some oil to flow to the cooler (radiator), and as the oil thins from heating up, more oil flows.   The GS radiator should be covered in quite cold weather to avoid over-cooling the oil.  The advantage of the GS plate is that it is simple, and takes up less room in the canister area, and since the frame on the GS models is higher, the thermostat unit would not fit those frames.  Take a look at a GS sometime.

For information in great detail about the oil filter canister, oil cooler system, etc., refer to:
https://bmwmotorcycletech.info/Oil.htm  
Also see:  https://bmwmotorcycletech.info/oilcansimple.htm

Two sketches, with notes, on the oiling system passageways, etc., are in   https://bmwmotorcycletech.info/oilsketch.htm
The notes contain information on many things dealing with the oiling system ...and some changes made by BMW.

The BMW airhead uses a wet sump oiling system:
That is, there is a container, called the sump, which most call the 'oil pan', that holds most of the oil, and plumbing/piping is arranged so that an oil pump draws oil from the sump to the oil pump for pressurized delivery in the engine oil passageways, etc. The type of pump design that BMW uses is called an "Eaton" ...which is a sort-of trochoid arrangement. The pump is very long lived, extremely powerful, and is capable of a enormous volume, hundreds of gallons per hour at high rpm. It is best not to remove the oil pump parts unless you have a reason to.  I do, however, remove the outer pump plate and take a few feeler-gauge measurements, and replace the oil seal, anytime the flywheel (clutch carrier in 1981+ models) is removed.  In the pump area there is an index mark to line up and the vane has one inner edge chamfered, which installs first. You MUST replace the O-ring if the plate is removed.  I use a wee drop of Loctite BLUE on each clean and dry screw or bolt and female thread.  The early plates, which used a screw and not a bolt, should be changed to the later type ...the plate groove depth was changed too, so change the early plate to later type, and use the later bolts.  There is more on this area a few paragraphs later....

Oil is lifted (sucked?) from the sump into a fixed non-moving pickup, and that pickup, which dips well down into the sump, has a coarse metal screen. There is a difference in the design of the early pickup and the late one.  The early model had a small gasket on each side of an adapter that is on the top of the oil screen unit; later models had the engine casting changed so only one gasket is needed, and there is no adapter. Be sure the bolts are tight and Loctited (blue), if you have the pan off for any reason. Inspect the entire filter pickup mounting and casting portions quite carefully to be sure there are no cracks.  Be sure ...if you have the spacer with the two bolts through it, that the bolts are tight.

Should you have your pan off, that is a good time to gaze up and inspect a cam lobe, marvel at the engine design robustness, understand what a tunnel-engine means. At that time, clean the screen, and be sure no cracks are seen in any part of the pickup and casting.  Be sure that the bolts are treated with a drop each of blue Loctite, & the bolts tight. If the spacer is plastic, replace it with the later metal version.  I use a very thin amount of Permatex non-hardening sealant on the pickup junction gasket(s).   For the pan gasket, no sealant is used, and the pan bolts must not be overly tightened, which will warp the gasket and cause leaks.  Tighten the pan bolts first, before removing the pan, to be sure none have stripped threads, that way you will know which threads to repair.  When replacing the gasket, the surfaces must be perfectly nick-free, free of any old gasket remnants, and you must tighten the bolts in small amounts in a cross-pattern; over and over until reaching specification torque.   Do NOT use a 75 ftlb torque wrench.  Use either a 4" wrench and feel for the correct torque, or, use an inch-pound wrench.  The torque should be 60 to 72 inch-pounds.  I use a bit higher torque when testing the threads BEFORE removing the pan. 80 INCH-pounds is fine.   After a few rides, recheck the 60 to 72 inch-pounds final tightening.   If you have a leak at the pan bolt below the oil canister area, it is acceptable to use non-hardening Permatex Form-A-Gasket goo on the threads, waverly washer, and underside of the bolt head.  https://bmwmotorcycletech.info/pangasket.htm

BMW has had at least three sizes of oil pans & various oil pickups; dipsticks;....etc.   Instead of listing all the variations of pans, pickups, ... and putting photos here, etc., Anton Largiader has done just about all of that already (but, see my engine internals article for the rest), and here is the hyperlink to Anton's article:  http://www.largiader.com/tech/oilpan/
I do have my own dipstick length and marking point measurements on this site:
https://bmwmotorcycletech.info/pangasket.htm

In instances of quite hot oil and very abrupt braking, the system may loose pressure for a moment, turning on the OIL lamp very briefly.  This is somewhat rare, and is unlikely on the later pans that have a forward-located surge baffle built into the pan.

The oil pump is located at the rear of the engine, to the right of the crankshaft, behind a 4 bolt (or screws on earliest models) cover plate. The center drive of the pump, which is via the rear end of the camshaft, is different on early and late models.  That cover plate has a rubber O-ring, and if you should have your flywheel (called a clutch carrier on the later models) out for such as a new main seal (and flywheel O-ring on later models), then you should remove the oil pump cover, install a new O-ring, oil it, and immediately replace the cover assembly. If the early model with slotted screws, replace the plate with the later model plate (which is re-dimensioned for the bolts and groove depth for the rubber O-ring!) , and use the later bolts, and, of course, use a new O-ring. As noted above, I use blue Loctite on the clean threads. Two types of O-rings have been used. The original was black, and a later type which is a few thousandths of an inch thicker, is red. Two types of plates were used, one for taper head phillips screws, and one for bolts. DO install the later cover with bolts, and a fresh O-ring, as the later cover O-ring groove has a changed dimension & stops leaking problems.   Allen bolts have also been used. The sort-of-trochoid 4 lobed oil pump vane is attached directly to the rear of your camshaft. That vane rotates inside of a 5 lobe rotating ring, and it may be hard to visualize how it works even if you have the cover off and are looking at it ...until the engine is rotated, while you watch the pump.  I suggest you search the Internet for a working rotating sketch of an Eaton oil pump. Here is one:
https://www.youtube.com/watch?v=OU9DZOmbGhk

The oil goes from the sump to the oil pump to the oil filter canister, where the oil is presented to the OUTSIDE of the oil filter, under very high pressure. The passageway (galley) from the pump to the oil filter canister area is plugged at the left side of the engine by allen plugs or screwdriver slot plugs, and are best left alone.  One such plug is quite visible, the other is rearward and inward a bit (around the corner, so to speak).  That is, you will see one plug on your engine below the oil pressure lamp switch ...horizontal with the left pushrod tubes.  The other is a bit to the rear, 90° around the corner, near where the transmission mates to the engine casting. Very high oil pressure can be in this galley at engine start-up, particularly if the oil is cold.   The oil passes through the oil filter element into the other engine oil galleys (cast-in piping).

NOTE: The oil pressure warning lamp switch is NOT located in the part of the left side galley I mentioned above.   Rather, that switch is located in a different part of the oiling system ...MUCH farther down the line, actually in the galley that supplies the rear main bearing.   The pressure in the switch area is about 14.5-29 psi at 800-1000 rpm; and about 60-74 psi at 4000 rpm.  These are official figures, and will vary rather a lot with oil temperature and oil specification.   The switch has threads of 12 x 1.5 mm.  NOTE that very early airheads had a 3/8 NPT thread ...also note that the later switch is the same as mid-seventies 2002 BMW car sender.

NOTE:  The timing chain is oiled from the output of the pressure relief valve, located at the front of the engine, internally, and it opens ~ 75 psi.  At very low rpm & with hot oil, there is minimal lubrication.  The chain itself wears little.  The upper sprocket located on the crankshift will wear.  It is common to call repairs in this area a "timing chain job", but usually the top sprocket and guide/tensioner are the worn parts ...but all worn parts in that area are replaced normally when needed.  There is an article on this website on that.... https://bmwmotorcycletech.info/timingchain.htm.  Because of the need for chain, guide, & sprockets oiling, it is not a good idea to idle the engine too slowly.  This is a worse thing to do with a well-worn engine.   Under 800 rpm is BAD, and 900+ is OK.   I tune all Airheads so hot engine idle is 1025 RPM.  Carburetors work better and are easier to adjust at that RPM; and initial throttle response is better too.

***There have been many changes to the oil filter area over the years. A considerable amount of confusion exists. DO NOT EVER GUESS at what you THINK should be fitted.   I have all the information you need on this this website at https://bmwmotorcycletech.info/Oil.htm.  Also read:  https://bmwmotorcycletech.info/oilcansimple.htm

***Haynes and Clymers manuals, with all their sketches, ARE WRONG in some details.   BMW sketches can also be wrong!   Keep in mind that BMW sketches almost always include every part that was ever used, and may not have the parts in the correct order of assembly.  When that happens, and it happens often on BMW parts sketches/lists, and when such as Haynes and Clymers use the same sketches, or similar to them ...confusion becomes worse.    If you make a mistake, and the previous owner or even BMW may have made a mistake, do NOT assume things were done properly, as a mistake may be very costly!

Do not use any outer gasket on the engine oil filter cover on any models not having the inner one-bolt filter cover ...unless your bike is in the 'exceptions'.  There are those exceptions, so read:   https://bmwmotorcycletech.info/Oil.htm

The early models with the one-bolt inner cap-cover are almost foolproof.

Do not be foolish, do be sure you totally understand the how's and what's and why's on the later canister setup; whether you have an oil cooler or not!

For models not having the inner single bolt cap cover, be sure there is a metal shim against the canister edge, unless your canister has a lip ...and even then a shim might be needed!   Read that above linked article!    Failure to do things correctly can cause $$$$ damage.

If the oil light ever comes on at idle rpm, you may have done damage already! ...except, perhaps, under severe braking, for an instant....  ...maybe.

There is that mentioned one bolt inner cover on earliest models and there are several types of outer covers, thermostat and non-thermostat covers, GS covers, metal canister shim, two basic types of internal pipes, many changes in O-rings and oil filter designs, use and non-use of a paper cover gasket, etc.  Do something wrong here and you might soon have a much reduced bank account.  NOTE that MANY BMW dealership shops do NOT understand Airheads!  Do NOT unnecessarily trust your BMW dealership mechanics to do a oil filter change properly.  You must KNOW that they have considerable Airhead experience!!!, and YOU CAN ASK about the finer details ...about those shims and O-rings!  I suggest you do it yourself.

The engine is lubricated by oil coming from the oil filter/canister area. If the filter is blocked, the bypass valve at the inner wall of the canister will allow the engine to continue to receive oil.  In a VERY rare instance a collapsed/failed oil filter has stopped oil from reaching the engine oil galley.  This may have been a poor aftermarket filter and/or too high an rpm on a cold engine when starting- up.  It is MY belief that the HINGED, BMW-SOLD filters are the BEST, and STRONGEST.    The primary outlet for the canister is the central pipe.   In the early non-cooler equipped bikes, the oil flows from the outside of the filter through the filter and then into the short center pipe, thence to the engine.  In the cooler and non-cooler later models, the center pipe is longer, and the pathway more convoluted, a look at the outer flange cover will show you the pathway.

For an oiling sketch with NOTES:    https://bmwmotorcycletech.info/oilsketch.htm
That sketch/page has a lot more information on it, be SURE to see that page! ...it includes a breather area photo you WILL want to see!

Things get complicated from the crankshaft main bearing area. There are SEVERAL routes for the oil from the bearing holder area. See the oil sketches, and there are arrows on the sketches to show flow direction.

1. An outlet goes to the TOP two LEFT cylinder studs. The engine casting base area of each of those two TOP studs, has a small hole. Oil travels from that hole outwards along the cylinder studs to the valve gear, and on its return from the cylinder head flows down the pushrod tubes to the sump, lubricating the cam and lifter during its passage through that area.  This is not a huge amount of oil, and the oil gets considerably heated by the cylinder head. The oil needs to be of high quality and contain ZDDP to protect the cam lobes and the associated lifters.

2. Similar stud oil hole outlets lubricate the RIGHT cylinder valve gear, & then back to the sump in the same manner.

3. Another outlet goes to the oil pressure switch and the crankshaft rear bearing. Pressure at the switch, with hot oil, is likely to be towards 30 psi at idle, and over twice that at high rpm.

4. Yet another outlet goes to the pressure relief valve, which opens at about 75 psi. That oil relief valve is located in the chain compartment at the front of the engine, it is a simple spring loaded plunger.   Since this point is WAY down the oiling system from the pump ...and oil has traveled through a lot of smallish passageways, it is notable that the pressure at the left side oil galley, the direct pump output, discussed previously, can be very much higher, especially with cold oil.

5. The crankshaft itself has a bore, fed from the same area, lubricating the rod big ends. Since the cylinders on a BMW airhead are NOT directly opposite, these are separate.

6. The piston rod piston ends (often called the little ends) are splash lubricated.

All the oil eventually travels via bearing clearance flow, valve gear flow, etc., back to the oil sump, and the process begins anew.

If your Airhead motorcycle has an oil cooler, some oil is routed from the oil filter canister area to the cooler radiator. That system uses a longer canister central tube and a special outer plate and o-rings, etc., to ensure that oil can flow to the cooler. A thermostat may be used in that outer cover (the GS uses a restrictive hole size in the cover).   Yes, I know I am repeating information.

***NOTE: on models before ~1979, oil routing was somewhat different to the front bearing, but this is of no real consequence.  It is shown in the oil sketch diagrams.  The exact serial numbers, dates, models, of the changeover are not remembered by me and I think it was phased into production so as to use up all the old parts/castings.

On the left side of all Airhead engines, below the starter cavity, to the left of the dipstick, is a small hole.  That hole is the drain for the starter cavity.   If a breather hose fails, oil may come out of that hole.  In the 1985/1986 and later models, a system of two electric solenoids, and a few other things, and a fuel tank modification, was standard on Airheads shipped to the U.S.  One solenoid positively shut off the fuel, petcock or not.  The other routed fumes from the fuel tank (perhaps the bike was in the sun, especially if parked?) into the crankcase, where the fumes could be sucked into the carburetors upon the engine starting via the breather system.   The fuel tank vent hose led to a short vertical vent pipe leading into the crankcase, and that pipe was to the left of the starter motor.  Many folks have removed the solenoids, one or both, which were mounted to the starter motor cavity area cover.   These folks may also have removed the flapper valve in the fuel tank that prevented a full load of fuel.  When the vent solenoid is removed, the vertical pipe going into the crankcase MUST BE PLUGGED, by bending over or a common tubing cap.  This site contains an article about the Pulse Air System, and those solenoids, etc:  https://bmwmotorcycletech.info/pulseair.htm

***A somewhat 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, of about 4 mm diameter, about 11 mm long, in the oil pan. The main bearing is a press-fit, but if the pin, which is supposed to be pressed-in and staked, comes out (substantial oil pressure is there, helping to push out the pin), then the bearing might rotate.  The pin is 11-11-1-253-184.   This is a SERIOUS event, and usually requires the entire front of the engine to be disassembled.

As mentioned much earlier herein, oil capacity depends on model and pan fitted.  Pan changes have been made for extra air volume for reduced oil consumption.  Later models have a drain-back hole in the bottom of the breather valve area, which additionally reduces oil consumption out the breather valve.  Later pans have surge baffles.

EXTERNAL oil filter conversions: