Oil Transfers....moving someplace other than where it belongs.
Water in the oil. Transmission input shaft seal.
Transmission and rear drive lubricants.
Installing the speedometer boot at the transmission.
© Copyright, 2012, R. Fleischer
1. If the REAR DRIVE has more oil; or, a reduction in the oil level, and the driveshaft oil level is changing, the seal at the rear drive input may need replacing. If the top (oil inlet) vent is spewing oil you may simply be in very cold weather, with a frozen-up breather vent. This does not apply, of course, to the driveshafts that are run dry in later models (such as GS)...assuming those driveshafts are not filling with oil from the transmission or someone has made the choice to put oil in the driveshaft!
It is possible for a clogged vent (the hollow speedometer cable bolt where the battery negative cable attaches) to allow the driveshaft housing to both pressurize or have a vacuum with a temperature change; such effects could allow a 'sucking' of oil forward, past the rear drive's input seal (or, a pressurized driveshaft housing forcing oil into the rear drive)....so this is an area to check first, as checking it is VERY simple, check the hollow bolt. THAT speedometer hollow bolt vent is NOT the vent being discussed just below.
Early models (/5 in particular) had more potential for driveshaft oil to appear to get by the rear drive input seal and fill the rear drive. On all early models (I do it on some later ones too!)....when I fit the pinion gear and nut, I put Permatex non-hardening Form-a-Gasket on the splines for the input gear (and Loctite BLUE on the threads where the nut tightens). It is possible for oil to get behind the splines of the input gear, behind the nut, and behind the seal containing outer threaded ring....and thus those areas have some sort of sealant applied. The oil travels right along those splined areas, getting past the seal. That nut must be VERY tight, see the torque specification page on this website.....and if it loosens, bad mechanical things happen. Later models have a thick plastic washer behind the input nut, and it deforms, so when removed, you see internal teeth from the splines in that washer. This washer seals the splines from oil passing to the drive innards. It is NOT on the early models.
The very earliest /5 models did not have a driveshaft and transmission venting system quite like the 1972+ models, and the rubber accordion could, if quite cold, reduce its size due to a slight vacuum in the driveshaft housing; and, be torn up some by the U-joint. A fix is possible. The same rubber accordion could expand rather a lot when the driveshaft housing was hot. See:
Now and then I hear of someone having a rear drive that has vented oil to the outside. This is USUALLY reported in the Wintertime. There can be several causes:
a. Rear brake drag is overheating the oil and it is spewing out the vent.
b. If the weather is very cold, the inner gears will throw the very cold oil upwards & tend to plug the
breather; or, moisture collected and the breather is frozen closed. It is OK to run the oil level half
an inch or so low, and when the rear drive heats up from normal riding, the phenomena stops.
c. If the input gear (pinion nut) is loose, not uncommonly seen, oil can work its way past the input seal,
from driveshaft to rear drive. If the nut loosens more than a little, the gears can be ruined, $$$$.
d. CHECK the rear drive oil level now and then. The original specification normal oil level is at the very
bottom of the threads of the filler plug on very early models. It is really much more than needed.
On later models, there is an oil level plug (extreme rear, centered, small hex plug).
That plug must not be over-tightened and.. proper oil level is such that the oil barely runs out, if
plug is removed.
The amount of oil is best measured and then installed. For the /5, 100cc, later, 150 cc. As little as 50 CC left in the driveshaft housing would likely be more than enough to lubricate everything, primarily the spline and gear cup to the rear drive, and it is questionable if anything much gets into the universal joint. The driveshaft level is hardly critical. Some folks simply measure the oil and install the measured amount. I do it that way. It is also possible, sometimes, to use a tiny screwdriver or a small allen wrench out of the on-bike tool kit as a measuring stick, but can be errors if the driveshaft angle is flatter and the only reliable method is measurement by volume. BTW, if the oil is put into the filler hole too rapidly, it will bubble up and overflow. Drizzle it in slowly.
It is rather common for oil to transfer TO THE TRANSMISSION. Sacked rear end will do that, as well as mountain riding, and a few other things. It is an annoyance, only. For really long tours, if I am riding a bike known to transfer oil to the transmission from the driveshaft housing, I keep a modest sized plastic syringe with a few inches of attached plastic line, in my tool kit...for transferring oil. You can improvise a wedge for the center stand. I have never seen, nor heard, of any airhead having the driveshaft run dry from this oil transference to the transmission. BTW: I am not worried about transferring oil that is molecularly somewhat sheared by transmission gears, back to the driveshaft. Or from any of the three places back to any other of those places. MORE on oil transfers INTO the transmission, in section 3., below.
2. If the transmission level has been lowering, it could be a faulty transmission output seal. It can also be the 13 mm headed bolt located in the center of the top of the transmission. Use a sealant on the bolt threads and its washer. I use Permatex non-hardening Form-a-Gasket. Oil is being sucked up into the air cleaner housing and then to the carburetors.
If you are doing an input spline
lubrication, and see the input seal of the transmission leaking at all...fix it
NOW!...it is NOT difficult.
Oil can move forward into the clutch area, due to
that leaking faulty transmission input seal.
Replacing the input seal CAN be done carefully, without doing too much other than removing the
transmission. Drill a small hole into the metal
part, insert a sheet metal or dry wall screw, and remove the seal. You can
do it with a fashioned puller and two such screws at 180 degrees. Several ways.
Just do NOT pry with a screwdriver and injure the sealing surface.
The seal that the shift lever goes through, into the transmission, fails now and then, and you do NOT have to remove the transmission to fix that.
The transmission of the early Paralever models (Paralever models have dry drive-shafts) have an open, not sealed, and too large, V shaped vent hole, located just forward of the transmission output flange (that's the part on the transmission output shaft that the U-joint bolts to)...it is...just above, at 12:00 position, of the transmission output seal (that seal, green, has the open end, spring end, facing rearward on the Paralever model). Paralever models should have that hole plugged with RTV silicone compound (or something similar), and then a very tiny hole put in it. Later models don't have it. NOTE what I said about the green seal...the Paralever models have the output seal (no matter color) put in REVERSED from the the other models.
3. A problem seen now and then is oil transfer from the wet type driveshaft to the transmission. The common complaint is something like: "I checked the oil in the transmission and it was overfilled, and the shaft housing was low by the same amount". For some folks this happens relatively quickly, few miles being needed...and for some only after long rides. What you can do is to put a thin piece of wood under the left leg of the center stand...which cants the bike slightly to the right. This wedge is for preventing oil from flowing out onto the ground. Unscrew the left side transmission filler/inspection plug, and remove some oil, I use a large-animal type plastic syringe with a bit of tubing forced onto the tip. I've seen up to 80 CC needing transfer. Move an estimated amount of oil from the transmission to the driveshaft; remove the wedge and tilt the bike back slowly, to avoid oil running out.... rechecking as you do so, the transmission oil level. Repeat until level is just at bottom of the transmission filler and oil check threads. The situation is not a serious problem, mostly it is an annoyance. It is almost impossible to fix this cheaply. Don't think about fancy venting ideas, they likely won't work.
Primary Causes for this transfer of driveshaft oil TO the transmission:
a. Sagged suspension.
b. Nivomat rear suspension units...which take a very short ride or a few bumps to self-pump up....these
were stock items on some RT models, optional on others. Nivomats after roughly 1982 pump up faster.
c. Plugged breather passages/holes: there are two. The one that sometimes gets plugged is the small
diameter hole in the special hollow bolt that holds the speedometer cable and battery negative wire. If
that hollow bolt plugs up, you will pressurize the driveshaft housing. If conditions (heat especially) are
bad enough, the accordion rubber will swell up. There is a bit of knowledge needed here, because on the
very early /5, mentioned earlier, there is no vent into the transmission. See below for description of the
other breather hole. NOTE that it IS possible for that wee hole to be plugged and have a VACUUM appear in the
transmission and driveshaft, and that can suck oil out of the rear drive, forward.
d. Considerable steep downhill riding, like in the mountains.
e. Heavy load...like 2-up or heavy rider or both.
f. Very bumpy roads.
g. Overfilled driveshaft housing. The amount of oil needed is actually quite small, 50 cc is enough for
lubrication of the rear drive input gear, 150 is the typical amount recommended for all after the /5 (use
100cc for it). The oil does not need, nor should it be, filled to 1/8" above the 'bell' located straight
down from the filler hole opening as sometimes recommended. BMW originally, on early models, said
to fill the oil level to, depending on where one finds the information, 'the filler plug'; OR, the lower
threads;...ETC. DO NOT. If you slide a small tiny screwdriver blade down through the filler port hole,
sideways alongside the internal bell, and see any oil at the tip when it is removed, you have plenty enough.
But, if the suspension has sacked out some, that way of measuring is not accurate. So, the best way to
check the level is probably to drain thoroughly, insert the proper amount of oil, and then, if you want to,
fashion your own measuring tool, for YOUR bike; although that may or may not work, if the tool cannot reach
the oil. NOTE that if the rear suspension is sacked out some, or otherwise you have the driveshaft on a
flatter angle, the oil is much more likely to be transferred to the transmission, particularly in downhill riding.
h. Combinations of the above 7 items.
i. NOTE: On all models it is IMPORTANT that the hollow bolt holding the speedometer cable (and
battery negative cable) at the right rear of the transmission, be open...that is, not plugged. Water gets
into the transmission via that hollow bolt...or...via a bad speedometer cable boot (even rainfall
does that) or, especially, by hose spraying during washing.... and it will destroy the
transmission bearings! NEVER spray water at this speedometer bolt and boot area when
washing the bike. See the control cables article #7b:
This is also the ONLY breather for the transmission (on most models) and the driveshaft. Clogging is
often only noticed in very hot weather, when the large accordion boot would swell up. The pressure
can force oil out of the boot. Never over-tighten this bolt, it may fracture. Snip a tiny section of the
battery lug, so you do not have to remove, only loosen, the bolt to disconnect the battery cable. There
is supposed to be washers on this bolt, two, and one is waverly; be sure you have them, and put
the battery lug between the flat one and transmission.
Note: The very earliest 4 speed transmissions did not have the vent at the rear of transmission, above the
seal. Thus the accordion boot does tend to swell-up, as the driveshaft area heats up. Leave it
alone, unless you love playing with things like drilling or machining holes in transmissions.
How to install the pesky speedometer cable rubber boot at the transmission output area:
REMOVE the hollow bolt that holds the battery cable lug. It also is the breather for the driveshaft
(and thereby the transmission on most Airheads). The hollow bolt SHOULD have TWO flat washers,
one under the lug, one outside the lug, and then a locking washer against the head of the bolt. You
can modify the lug by snipping it JUST enough to let it push over the hollow bolt, which allows you to
disconnect the battery without completely unscrewing the hollow bolt. By using the washers as
noted, you will NOT spread the cut lug as you lightly tighten the hollow bolt. NEVER over-tighten
that hollow bolt.
Method #1: Push the rubber back on itself and push up onto the cable with the large end upwards
(pushed back on itself). You will then be pulling the large end downward, so the small
end stays upwards. I lubricate the cable and the top of the rubber (inside and outside)
with copious amounts of silicone grease, and after it is installed and the large end pulled
Method #2: Use a taper tool to expand the cable end of the boot, you could make a tool, but you
will find a caulking tube tapered sealing cap works fine.
Method #3: Install a piece of shrink tubing, and lubricate it with one drop of dish detergent, push the
boot up. You can do this without the tubing too.
It is critical that the rubber boot does not leak water into the transmission. This has been the cause
for a lot of $$$ repairs. The typical result along the way is transmission oil that looks like coffee with
cream. My suggestion is that you either put a fair amount of grease inside the boot; or, you seal the
boot (my preferred method). Sealing the boot is simple:
Push the top of the boot down a bit, so as to expose a bit more of the cable sheath. CLEAN
the sheath with a bit of acetone or other fast drying solvent, using a rag. Put a small amount
of black RTV sealant at the top, smooth it nicely with your finger, and then slide the top up a
bit, and then reseal again. The typical leaky place is the actual top of the rubber boot,
where the cable rises upwards....the top expands with age. That is why I use RTV at
Some folks use a zip tie. That can work, but there CAN be problems. This is especially so if the zip
tie is a larger one. What happens is that tightening of the zip tie makes the top of the boot squeeze
OUT OF ROUND, and can actually make things WORSE for water getting inside, and this CAN
happen after awhile, long after you forgot to recheck.
After the RTV cures (allow overnight), use something like Armor-all or 303, etc., on the entire boot.
This greatly prolongs boot life, as the rubber compound BMW uses is adversely affected by smog
80W90 GL5 hypoid differential oil, NON-synthetic, is what BMW specified...and the SAME for rear
drive, transmission, driveshaft (non-Paralever). SOME who use synthetic oils find they have leaks.
IF you use synthetic gear oils, and have leaks, I suggest you switch back to petroleum oils, and the
seals may fix themselves, after a considerable number of miles. I am not against synthetic oils.
In fact, use of them in the transmission is LIKELY to prolong the transmission life.
You do not have to use 80W90. You can also use 75W90. There is a wider-range (85W140) oil, but
I suggest that you DO NOT use them.... it is probably only better for those in super hot weather and
very high speeds or loads, etc). DO NOT willy-nilly change to 85W140!!! MY BELIEF is that it is
NOT at all any better until the rear drive is exceedingly hot. At medium situations, it likely is WORSE.
04/21/2003: add .htm title; clarifications (minor)
02/04 and 06/2004: minor clarifications
03/06/2005: incorporate all changes and updates and fix typos
01/21/2010: update, and review entire article
06/14/2012: Revise 9
10/12/2012: Add QR code; add language button; update Google Ad-Sense code; minor other
04/30/2015: Update article
© Copyright, 2012, R. Fleischer
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