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Oil Coolers
Copyright, 2012, R. Fleischer


WHY an oil cooler?
Air cooled motorcycles can run quite hot in some situations.   This includes large amounts of throttle/power for long periods of time; pulling a trailer or sidecar where a lot of engine power is needed; extremely bad traffic conditions; all made worse in quite hot weather.  Your BMW Airhead engine has more than enough cylinder fins & head fins to dissipate enough engine heat in most circumstances, including some fairly extensive stop & go traffic, & modest speeds pulling a trailer or sidecar.   BUT, there are limits.

Engine oil should operate in a reasonable temperature range after the engine has warmed up; not too cold, not too hot.  Usually, a multi-grade oil such as 20W50 is used in our Airheads.  Quality 20W50 MOTORCYCLE oil is far better than the old single grades type of oils, & is quite adequate for most conditions, but a thinner oil might be wise if temperatures are at or below freezing; perhaps a 10W40.   10W50 and 15W50 oils are also available.   Further information is here:  as well as my various oil articles, including and
Please refer to all these various articles, there are at least 5.

Engines operate best, over-all, when their motor oils are ~180 to ~235F.   Oil is MUCH hotter than that at the valve guides & other areas of the cylinder heads, so the oil is exposed to quite high temperatures in some engine places.  That usually means some deterioration of the oil, from evaporation (particularly of additives) to some form of deposits including carbon or similar, which is not good.  Because oil (and especially its additives) deteriorates with usage, oil is changed regularly....depending on miles, time, & conditions of use.   You need a high enough oil operating temperature to ensure good oil flow & lubrication, but if too high, the oil deteriorates faster. It needs to be high enough to evaporate moisture that has accumulated.  Petroleum oils start to increasingly deteriorate above 235F.   The deterioration rate is a relatively steep curve, & is VERY bad at perhaps 300F.  For this, and a few other reasons, some folks use part-synthetic or full synthetic oils.  

The Airheads started with the /5 series, production began in late 1969.  Oil coolers were not fitted, and were generally not needed.   As time went on engine sizes grew; power output increased & fairings were installed on some models.  More engine heat was developed or trapped.  While kits to install oil coolers were available early-on for those having special needs, it was not until 1978 that BMW began incorporating them into production models, with the R100S, R100RS & R100RT.      Many have converted other, especially earlier models, to have oil coolers.  The faired models, especially the RT, do not get as much cooling as the naked models, so coolers are more valuable for the RT.   
BUT...oil coolers are not at all always necessary.  Contrary to some opinions, the oil does not usually get extremely hot in city driving....because low power is usually being used, as opposed to high power for high speeds.  In more severe conditions, stop and go, hot weather, high power output, the oil MIGHT get hot enough to benefit from an oil cooler. Thus, BMW incorporated an oil cooler.

Description of the BMW cooler setup:
The BMW factory oil cooler is a finned radiator that mounts to the front area of the motorcycle.  The cooling medium is the oncoming air.  An oil filter chamber outer cover plate of one of two BASIC types available (thermostat or non-thermostat) replaces the original non-cooler equipped motorcycle's outer oil filter chamber cover.  The internal 'pipe' over which the oil filter element is installed, is changed in length. In early production Airheads, you also needed to change the canister itself when converting to a cooler.  Two special hoses connect the cover to the radiator.   On the models with thermostats, the thermostat starts to allow SOME oil to go from the filter compartment to and through the oil radiator, at about 195F.  The oil passage is fully open when not very much hotter.  On the GS models & some others, the thermostat plate was not used, in favor of a different plate.  This GS plate better fits those models where the frame tube might be in the way to fit the thermostat version.  Instead of a thermostat, a simple bypass hole is used.  Because there is no thermostat on these particular models, in very cold weather, BMW recommended that a COVER be placed over the radiator to prevent over-cooling, & BMW sold such covers at one time, maybe still does; but last time I looked at the fiche I did not find it.

The details:
BMW issued various cooler bulletins, which are called a Service Information sheets, but each is known to all BMW wrenches as an "SI", but the most pertinent version for our purposes in this article was in September of 1980.  The SI was #11-016-80 (2019).  This was a fairly extensive bulletin about fitting a cooler to early models. At that time, BMW offered a kit, it was #11-42-1-335-396.  The kit was still available when I last checked in May 2016, and was OVER $1100.00 (in 1995, it was $288.00).  The installation was a bit of work, & required a canister sleeve #11-11-1-263-343, now 11-11-1-338-203.  Most folks will install many parts for a oil cooler installation, where one did not exist previously, from a wrecked or otherwise salvaged Airhead.  It is entirely possibly to install either outer cover of the two designs into ANY of the Airheads, provided there is right side frame clearance, usually only a problem on some GS models, so the GS-style cooler cover plate is used with those, and the thermostat types with others, although not mandatory.

Installing an oil cooler into an early bike that had the early style filter arrangement, which is identified easily by it having a flat plate outer cover and an  INTERNAL ONE BOLT CAP, is a fair amount of work.   You first drained the engine oil, then removed the pan.  You had to remove the oil canister via the pan & the filter outer cover hole; it was often quite a bit of a chore to get it loose & removed.  You had to remove the perforated center tube.  To install the canister, BMW wanted you to make a special tool; the SI had the exact dimensions for that tool.  You had to install the new canister sleeve (tube) to a precise depth, such that the outer edge was 3.0 +- 0.4 mm at the engine cover plate boss area.   You had to clean up the center pipe threads in the engine and install the new long pipe, using Loctite RED on the threads.  It was a good idea to check the bypass valve at the end of the oil chamber area.

It was a good idea to make a special mandrel tool to install the center pipe.  BMW did not mention that.   It is possible to use a very large & specially ground VERY thick blade screwdriver to do that.  I prefer making a REAL mandrel tool to avoid problems.

The original kit utilized straight filters (no hinged ones were available then).  The kit had 2 small red O-rings of rubber material (that were not used nor needed with later filters that had bonded rubber at one or both ends).  The new outer plate used a medium-sized round rubber O-ring that eventually was replaced with a square sectioned one put inside all later filter boxes.   The large O-ring for the outer cover, a very critical one, was eventually a white one.  The white O-ring is still used to this day. This white O-ring MUST be replaced every time the outer cover is removed (usually only for filter replacement). The white O-ring seals the canister to the engine casting in TWO ways, so to prevent oil from the canister leaking into the crankcase, and to prevent it leaking at the outer flange cover also. The original kit required the horn to be moved & special bracketry installed (& some made up).
The cooler fittings at that time were:
07-11-9-901-682, screws, 3 needed
11-42-1-336-904 cover
11-42-1-335-987 angle piece
11-42-1-335-986 holder
11-42-1-336-936 grommets, 2 needed.

For practical purposes now, especially considering the cost for new parts, you would get a salvaged radiator cooler (& all of the other parts you could), of either the thermostat or non-thermostat type.  If you had a EARLY model with the internal cap cover, you needed to convert the central tube & canister.  OTHERWISE,  with later models, all you need is the outer cover, hoses, cooler, & brackets from a salvaged bike.

GS non-thermostat covers had a bypass hole, easily seen when looking at the inside of the cover.   Early ones were made with a 2 mm bypass hole. That hole is too small.  If yours is 2 mm; drill it out to 4 mm.  You can use a 5/32" drill bit.  Clean up your work...leave no sharp edges nor any metal particles.

Articles on this website deal with the so-called $2000 O-ring.  You want to measure your canister depth during installation & install one or more shims (not always, some very late canisters have a lip & may or may not require one or more shims for proper pressure on the sealing O-ring) & paper gasket (seldom). If you have not read the  article, please do. It contains a LOT of information about the oil system; the various filters, the distances you need to measure, etc.  There are other oil & system articles on this website.  Read them!  There is a reason this paragraph is in large type & colored red!

There is an oil bypass valve located at the innermost end of the oil filter metal canister on ALL models of Airheads.  The purpose of this spring-loaded ball-check valve device is to allow oil, unfiltered, to get into the engine, from the pump & then canister, if the filter in the canister somehow is blocked from oil flow.   This ball-check valve has very rarely come loose, & you find parts in the canister area.  Somewhat 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.  If you have to replace the valve or otherwise repair it, 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 & where it seats.  There is no specification on how deep to install the slotted holding part, so you will have to estimate NOT screw it in way too far, you will change pressure characteristics.   This caution paragraph is repeated elsewhere's in this website, & you can get a better idea of the oiling system by going to:   If you replace the valve, allow the BLUE Loctite to set-up for a full day minimum, before running the engine.

What about EXternal oil filters?
That is covered in my article 51A.

Initial release:  07/26/2007 with very little change on 01/03/2009.
02/24/2011:  now is 50D
05/20/2012:  add reference to article 51A
09/24/2012:  Some editing for clarity!!; add QR code; add Language button; update Google code
2013         :  Language button code was removed due to scripts problems with some browsers.
02/05/2016:  Increase font size; better left justification; less colors; update meta codes, minor clarifications.
05/25/2016:  Final update on meta-codes, scripts, layout, some clarifications.


Copyright, 2012, R. Fleischer

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Last check/edit: Wednesday, May 25, 2016