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Oil Coolers
Copyright 2021, 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; which is common when traveling at high speeds.  Other instances will include pulling a trailer or having a sidecar where a lot of engine power is needed.  Constant high speeds raise engine and oil temperature CONSIDERABLY.  Certain types of bad traffic conditions can also cause high oil temperature, as well as very high cylinder head temperatures.   Traffic stop and go in cities, and slow traffic on freeways are not usually a problem.  Use of fairings in general (particularly early versions of BMW Airheads RS/RT fairing lower center pieces that are not ventilated) can cause heat problems.   Engine cooling is obviously worse in quite hot weather, but your body is much more sensitive to this than the engine!  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 be in an approved 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 single grade oils. A thinner oil might be wise if temperatures are near, at, or below freezing; perhaps a 10W40.   10W50 and 15W50 oils are available.   Further information is here:  as well as my various oil articles, including and  Please refer to these various articles, there are at least 5.  See section 50 and 51 items on this website, available on the Technical Articles List page.

Motor oil in your BMW Airhead engine works best when it is ~180 to ~235F in the engine oil pan.   The oil is considerably hotter at the valve guides & other areas of the cylinder heads where a small amount of oil circulates slowly at very hot places.  There is some deterioration of the oil, from evaporation (particularly of oil additives absolutely needed and so part of the oil you purchase).  Certain types of combustion products also age the oil, and even leave deposits, typically carbon. Overall, the deterioration is slow, mostly because only small amounts of the oil are exposed to high temperature for any period of time. Because oil (and especially its additives) deteriorate with use, oil is changed regularly ....depending on miles, time, & conditions of use.   You need a high enough oil operating temperature to ensure good oil flow & good lubrication, but if the temperature of the oil is considerably too high, the oil deteriorates rather fast, and even typically gets 'burned' 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 to the effectiveness.  Petroleum oils 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 for normal riding situations.   As time went on engine sizes grew; power output increased & fairings were installed on some models.  More engine heat was developed & trapped from these things, particularly at higher speeds.  While kits to install oil coolers were available early-on for those having special needs, such as for racing, 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 standard 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 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 on some models especially so-affected, such as the faired models, and in some sport performance models, like the R100S.  The benefits from using an oil cooler when appropriate is not only good for lubrication purposes and extending the oil additives life, but it helps lower excessive engine temperatures, decreasing wear.  Although not often thought of in this way, the Airhead engine is both air-cooled and partially oil-cooled.  Even though the effect of the BMW oil cooler on oil temperature is modest, the over-all effects on the oil and wear, etc., are fairly high.   Note that system oil that is too hot by even 10 or 15 F, is helped considerably by a cooler that lowers oil temperature that amount.   Over-all, the oil needs to have many proper characteristics, but the basics are proper viscosity for atmospheric temperatures expected, a viscosity index that ensures reasonable viscosity over the expected temperature range, additives that do many things.....and this list could go on and on.   Lubrication theory, presented in my oil essay article, is ...or can be...very complicated.     You, as the owner/operator, need only to use a good oil, rated for your motorcycle, and change it on a proper schedule.    Common automotive oils do NOT meet the needs of your Airhead motorcycle!  This is especially due to the type of cam and cam follower, used on your Airhead.   There are many cars that were made decades ago that have the same oil requirements as your Airhead.

Description of the BMW cooler as installed by the factory:

The Airhead motorcycle was not originally designed for all to have an oil cooler.  At least, that is what I was told.  To incorporate an automotive style of easily removable filter could have been done, but there was no need, then.  The filter system as designed was simple, protected by an inside-of-engine location (the Germans like things neatly done), and did the job required.   For those who raced or who had special requirements, for the /5, BMW engineered a 'reasonably simple' oil cooler as an add-on.   That morphed into a more complex installation some years later, which has caused numerous $$$ problems, ....caused by owners, and even supposedly knowledgeable repair technicians, ....who did not understand the design, operation, and important cautions.

BMW never published detailed information on how the owner/rider ...and in many instances enough shop information....should maintain this area of the motorcycle.   The measurements, assembly, when to use a paper gasket, when to have 1 or more special shims, and the variety of various filters over the years, ETC., has made the 'oil filter changes' somewhat difficult to understand and deal with, and badly difficult for some.   The information in this website is intended to be 100% fully informational on the subject, in every which way.

BMW did not incorporate a simple automotive style of spin-on filter on any of its motorcycles until 1985, with the introduction of the K100.  The Airheads have never had such a filter from the factory, primarily due to the re-engineering and re-tooling expense.  It has been done by aftermarket suppliers by external add-ons, but with complications and considerable expense, and likely NOT worth the expense and bother, as the oil can be rather cheaply just changed more often for severe conditions use.   For the stock, no-cooler, BMW Airheads, especially the earliest models, the oil filter chamber and R/R procedure is simple, and pretty much fool-proof.   NOT SO the later type, cooler or not.

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 plate.  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.  All cooler-equipped Airheads utilize two hoses (and may have metal pipes besides the rubber hoses) to connect the new style filter chamber plate cover to the cooling radiator.   On models with thermostatic covers, the thermostat begins 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 very much hotter.  On the GS models & some others, with frame tube interference, 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, BMW used a simple properly sized bypass hole in the cover plate.  Because there is no thermostat on the GS 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 could not find it. Anyone with a wee bit of sewing experience can make a decent cover.  The thermostatic plate and non-thermostatic plates will interchange, if enough room to the frame tubing exists.   You do not, however, want to have to remove or tilt the engine, nor exhaust system items, in order to access the filter!  Some right side exhaust header pipes came with a dimple to give a bit better clearance.

The details:

BMW issued various Service Information Bulletins for the cooler. These are known to all BMW professionals as an "SI".  If you think that BMW's SI's cover all you need to know are WRONG!

The most pertinent SI for our purposes in this article was dated 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; but it is NLA now; although all the items are.  NOTE:  some on-line fiche does not work in search mode if there are hyphens in the parts being looked for.  Thus, use 11421335396.   Use that advice for all BMW parts.  If you DID find and install the kit, the installation was more than a bit of work, & required a canister sleeve #11-11-1-263-343, now modified by the factory to include a better outer edge, possibly eliminating use of any shim(s): 11-11-1-338-203.  Most folks will install parts as needed for a oil cooler installation from a wrecked or otherwise salvaged Airhead.  It is entirely possibly to install either outer cover of the two basic 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.  Note that there are several versions of outer plates for NON-cooler Airheads.   Get acquainted with them, by looking at various fiche; usually the quick glance method at the fiche listings for the various models (do try 11421335396) will show which section has the most items shown....and has more information, perhaps on cover variations too.

Installing an oil cooler into an early Airhead motorcycle (/5...) 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 an extra considerable amount of work.   You first drained the engine oil, then torqued the pan bolts to about 80 inch-ounces to see if any threads needed to be repaired after removing the pan. Cleaning the case surface where the pan gasket was fitted can be a bit or work. You had to remove the oil canister via the pan being removed for internal access & access to the filter outer cover hole; it was often quite a bit of a chore to get it loose & removed. I used a very large torch flame to expand the case where the canister is pressed-into.   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 confuse you with the center tube) to a precise depth.   The outer edge was to be 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 is 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...yes, it is worth the effort of making a tool (ask a machinist to make one for you).

The original kit utilized straight filters (no 'hinged' filters were available then).  The kit had 2 small red rubber material O-rings (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 (all BMW retail-sold filters came in 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 removed 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 might 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, the website you are very likely reading this on, 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. Notice the capitalized O in Oil.htm.  That article 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.  ESPECIALLY be cautious with "advice".  BE SURE you understand how it all works, and, thus, you will understand when people are offering BAD advice.

There is an oil bypass valve located at the innermost end of the oil filter metal canister area 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 might find its 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.  Consider oiling the ball area by spraying oil at it, after installing the ball and spring...that also may prevent any 'stray' Loctite from promoting ball-sticking.   The spring design changed, info is in this website.

External oil filters are 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.
11/27/2020:  Updated.
05/05/2021:  Clarifications and cleanup, only.

Copyright 2021, R. Fleischer

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Last check/edit: Wednesday, July 07, 2021