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Oil Coolers
Copyright 2017, 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; use of fairings in general (particularly early versions of BMW Airheads RS/RT fairing lower center pieces that are not ventilated).   Engine cooling is obviously 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.

For proper engine operation, lubrication, protection, etc., engine oil should operate in a reasonable temperature range after the engine has warmed up; not too cold, not too hot.  The oil also needs proper viscosity over its entire expected temperature range.  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 near, 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.

Motor oil in your BMW Airhead work best when it is ~180 to ~235F. As a general rule, that means the temperature in the engine oil pan.   Oil is considerably hotter at the valve guides & other areas of the cylinder heads where a small amount of oil circulates at very hot places, and there are other areas the oil may be hotter than in the oil pan.  The result is that 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. Overall, that deterioration is quite slow because only small amounts of the oil are exposed to high temperature for any period of time. 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. Bulk oil temperature needs to be high enough to evaporate the vast majority of moisture that has accumulated.  That accumulation happens at every start-up and cool-down. Some oil additives help with moisture problems, but there are limits; and, most additives tend to evaporate over time and temperature.  Petroleum oils start to increasingly deteriorate above 235F.   The deterioration rate is on a relatively steep curve, & is very bad by perhaps 300F.  For this, and a few other reasons, many folks use part-synthetic or full synthetic oils. Because the required additives evaporate and deteriorate over time and temperature, oil change intervals can not be overly extended by using synthetic oils.

Airhead production began in late 1969 with the /5 series.  Oil coolers were not fitted, and were generally not needed, as there was plenty of fins, etc., to enable adequate cooling.   As time went on engine sizes grew; power output increased & fairings were installed on some models.  More engine heat was developed or trapped from these things.  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 over-all cooling as the naked models, so coolers are used for the RT (and RS).    BUT...oil coolers are not always necessary.  Contrary to some opinions, the engine oil does not usually get extremely/excessively hot in most city driving ....because low power is usually being used, as opposed to high power for high speeds.  In more severe conditions, such as extended stop and go, quite hot weather, and/or high power output, the oil (and the motorcycle engine) can get hot enough to benefit from an oil cooler. Thus, BMW incorporated an oil cooler.  The benefits from using an oil cooler when appropriate is not only good for lubrication purposes but it helps remove excessive engine temperatures, decreasing wear.  Although not thought of in this way, the Airhead engine is air-cooled and partially oil-cooled (via air passage).  Even though the effect of the oil cooler on oil temperature is modest, the over-all effect on the oil and wear, etc., is fairly large.

Description of the BMW cooler as installed by the factory:

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 (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, as is the oil filter itself. In early production Airheads, if you were installing a cooler you also needed to change the canister itself when converting.  Two hoses connect the new style filter chamber cover to the radiator.   On models with thermostatic covers, the thermostat starts to allow some oil to go from the filter compartment to and through the oil radiator, at about 195F.  The thermostat is fully open when not much hotter.  On the GS models & some others, a thermostatic plate was not used, in favor of a different plate.  The 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 thermostatic plate and non-thermostatic plates will interchange, if the room exists.

The details:

BMW issued various Service Information Bulletins for the cooler. These are known to all BMW professionals as an "SI".  The most pertinent version for our purposes in this article was dated 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;  #11-42-1-335-396.  The kit was still available when I checked in December 2017, at $1120.00.  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 parts as needed 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.  If you are doing an installation, you may want to consider the GS type plate.

Installing an oil cooler into an early Airhead motorcycle that has 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 considerable 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 filters 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 part, 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 Airhead with the internal cap cover, you needed to convert the central tube & canister; otherwise, with later models, you need 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.

An article on my website has extensive information about the so-called $2000 O-ring.  You must measure your canister depth during canister 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) & seldom a paper gasket. It is a good idea to measure even the factory installed canister now and then, to be sure it has not moved inwards.  If you have not read, please do so. 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!  Quite serious damage to your engine is possible by not properly dealing with the canister depth, shim, and O-ring. Do not depend on BMW literature or other publications.

There is an oil bypass valve located at the innermost end of the oil filter metal canister on all models of Airheads, cooler system or not.  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 area, 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, which is 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 is repeated elsewhere's in my 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.
08/31/2017:  Go over entire article.  Reduce changes to fonts and colors; clean-up layout.

Copyright 2017, R. Fleischer

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Last check/edit: Monday, July 22, 2019