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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.     Generally speaking, your BMW Airhead engine has more than enough cylinder fins and head fins to dissipate enough engine heat in most circumstances, including some fairly extensive stop and go traffic, and modest speeds pulling a trailer or sidecar.  

    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 multigrade oil such as 20W50 is used in our Airheads.  Quality 20W50 MOTORCYCLE oil is far better than the old single grade type of oil, and is quite adequate for most conditions, but a thinner oil might be wise if temperatures are much 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.

Motor oils operate best at about 180-235F (approximately).   Oil is MUCH hotter than that at the valve guides and other areas of the cylinder heads, so the oil is exposed to quite high temperatures after engine warm-up.  That usually means some deterioration of some (or, all, weakly, depending on your viewpoint) of the oil to a form of various deposits including carbon or similar, which is not good.  Because oil deteriorates with usage, it is changed regularly....depending on miles, time, & conditions of use.   You need a high enough oil operating temperature to ensure good oil flow and lubrication, and if too high, the oil deteriorates faster. It needs to be high enough to evaporate moisture that has accumulated.  Petroleum oils start to deteriorate above 235F.   The deterioration rate is a relatively steep curve, and is very bad at perhaps 300F.  For this, and a few other reasons, some folks use part 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, and power output increased, and fairings were installed on some models...all of which meant more engine heat was developed.  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 and R100RT.      Many have converted other, especially earlier models, to have oil coolers.   They are not at all always necessary.

Description of the BMW cooler setup:
    The BMW factory oil cooler is a finned radiator that mounts to the front area of the motorcycle.   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 the early production of the Airhead, you 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.   It is fully open when not very much hotter.   The system is a BYPASS, it is NOT a full-flow through the radiator.    On the GS models, and some others, the thermostat plate was not used, in favor of a different plate; and 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 hole bypass is used.  Because of that, in very cold weather, BMW recommends that a COVER be placed over the radiator, and BMW sold such covers at one time, maybe still does.

The details:
BMW issued a bulletin, which is called a Service Information sheet, but known to all BMW wrenches as an "SI", 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 September 2014, and was OVER $1000.00.   The installation was a bit of work, and required a canister sleeve #11-11-1-263-343, now 11-11-1-338-203.  Most folks will install a oil cooler from a wrecked or otherwise salvaged Airhead if they want one.

    To install an oil cooler into an early bike that had the early style filter arrangement (which is identified easily by it having an INTERNAL ONE BOLT CAP), was 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 and the filter outer cover hole, and it was sometimes a bit of a chore to get it loose and removed.  You had to remove the perforated center tube.  To install the canister, BMW wanted you to make a special tool, and 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 from the engine cover plate boss area.   You had to clean up the center pipe threads in the engine, and on the new pipe... very carefully, ...leave them dead clean, and then install the new long center pipe, using Loctite RED on the threads. 

    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 and specially ground thick blade screwdriver to do that.  I prefer making a REAL mandrel tool.

    The original kit utilized straight filters (no hinged ones were available then), and 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.  These square and white O-rings are still used to this day for the coolers, and to seal the canister and engine on the the later non-coolers models too.  The original kit required the horn to be moved and special bracketry installed (and 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, you would get a salvaged radiator cooler (and all of the other parts if you could), of either the thermostat or non-thermostat type and simply install them.  If you had a EARLY model with the internal cap cover, you needed to convert the central tube and the canister.  OTHERWISE,  with later models, all you need is the outer cover, hoses, cooler, and brackets from a salvaged bike.

NOTE!   Some early non-thermostat covers, used on the GS models, were made with a 2 mm bypass hole. That hole is very visible when you turn the cover upside down.    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, and install one or two shims (not always, some very late canisters have a lip and may or may not require a shim for proper pressure on the sealing O-ring) and paper gasket (seldom). If you have not read the  article, please do. It contains a LOT of information about the oil system, and 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 and colored red!

    There is an oil pressure 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 pass into the engine from the pump and then canister, if the filter in the canister somehow is blocked from oil flow.   This ball-check valve has very rarely come loose, and 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....very 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, and then apply BLUE (medium strength) Loctite or equivalent, in a SMALL AMOUNT to the threads.   DO NOT get any on the ball and 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, and 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, 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
In 2013 the language button code was removed.


Copyright, 2012, R. Fleischer

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