© 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 grade type of oil, & is quite
adequate for most conditions, but a thinner oil might be wise if temperatures are at or much
below freezing; perhaps a 10W40. 10W50 and 15W50 oils are also available. Further
information is here http://bmwmotorcycletech.info/coldstarting.htm as well as my various
oil articles, including http://bmwmotorcycletech.info/viscosity.htm and
Please refer to all these various articles, there are at least 5.
Motor oils operate best at about 180-235°F (approximately). 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 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 (and especially its additives) 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 & 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 deteriorate above 235°F. The deterioration rate
is a relatively steep curve, & is VERY bad at perhaps 300°F. 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...all of which meant 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. 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.
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 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 195°F. It 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 hole bypass is used. Because of there not being any
thermostat, in very cold weather, BMW recommends that a COVER be placed
over the radiator, & BMW sold such covers at one time, maybe still does; but
last time I looked at the fiche, did not find it.
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 February
2016, and was OVER $1200.00. 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 a oil cooler, where
one did not exist previously, from a wrecked or otherwise salvaged Airhead. It is entirely
possibly to install ANY of the cooler 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.
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 & 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, & on the new pipe... very carefully, ...leave them dead clean; 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 & 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 (and some made up).
The cooler fittings at that time were:
07-11-9-901-682, screws, 3 needed
11-42-1-335-987 angle piece
11-42-1-336-936 grommets, 2 needed.
For practical purposes now, you would get a salvaged radiator cooler (& all of the
other parts if you could), of either the thermostat or non-thermostat type & install them.
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, & 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 http://bmwmotorcycletech.info/Oil.htm
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 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 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....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, & 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 ....do 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, 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.
02/05/2016: Increase font size; better left justification; less colors; update meta codes, minor clarifications.
© Copyright, 2012, R. Fleischer
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