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Oil Transfers:
...oil moving someplace other than where it belongs.
Water in the transmission oil.  Transmission input
shaft seal. Transmission & rear drive lubricants.
Installing speedometer boot at the transmission.

Copyright 2020, R. Fleischer
article 49

1.  If the rear drive has excess oil; or, a reduction in oil level; and the driveshaft oil level is changing up or down, the input seal at the rear drive may need replacing.  It is also not uncommon to find the rear drive input nut loosened...DO NOT WAIT....fix this immediately!

If the vent located at the top of the rear drive is spewing oil you may simply be starting up in very cold weather, possibly with a frozen-up breather vent; more on that later, below; or using the wrong gear oil viscosity and quality.  Later model driveshafts that are run dry, such as GS should remain dry.  More a few paragraphs below.

It is possible for a clogged vent (the hollow speedometer cable bolt where the battery negative cable attaches) to allow the driveshaft housing to either pressurize or have a vacuum with temperature changes.   Such effects could allow a 'sucking' of oil forward, past the rear drive's input seal (or, a pressurized driveshaft housing can force oil into the rear drive) this is an area to check first, as checking it is very simple, just 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 get past the rear drive input seal and get into 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, a medium strength Loctite, on the threads where the nut tightens).

It is possible for oil to move, via the splines of the input, to behind the nut, ...and to behind the hidden-seal-containing-the-outer-threaded ring ....and thus those areas have some sort of sealant or gasketing liquid applied by me.  I use Permatex Form-a-Gasket non-hardening version ON THE SPLINES.  Otherwise, oil may travel along the splined area, getting past the seal.

The rear drive input nut must be torqued very tight.  I heat the entire rear drive in an oven, for removing & replacing the threaded ring.  See the torque specification page on this website for details, which includes using Hylomar on the threaded ring, & Loctite on the nut threads:
Scan down to section 33-.

Later models have a thick plastic washer behind the input nut.  During assembly the plastic deforms, so when removed, you see internal teeth formed from the splines in that washer.  This washer seals the splines from oil passing to the drive innards.  It was not used until later models.  No, I have not tried installing it into earlier models.

If the nut loosens, BAD mechanical things happen to the rear drive innards.

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 rubbed some by the U-joint.  A fix is possible.  The same rubber accordion could expand rather a lot when the driveshaft housing was hot. The heating and cooling made for easier oil leaks and oil movements. See:

See 3.  well below

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; or, otherwise cold weather.   There can be several causes:

Rear brake drag is overheating the oil and it is spewing out the vent.

If the weather is very cold, the inner gears will throw the very cold oil upwards & tend to plug the breather; or, moisture may have 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.

If the input gear (pinion nut) is loose, rather commonly 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, $$$$.

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. That is more than needed.  On later models, there is an oil level inspection plug (extreme rear, centered, small hex plug).  That small inspection plug must not be over-tightened and the proper oil level is such that the oil barely runs out, or not quite, if the plug is removed.  Some fill the rear drive from that inspection plug, which is OK, but you must dribble it in slowly, or it will burp back at you and make a mess.  Most fill the rear drive by measuring the amount, installing it by removing the top venting plug and thus they can disregard the troublesome weak threads on the rear inspection plug.  I am careful, and I do use the plug level.

Like the rear drive, the amount of oil for the driveshaft is, perhaps, best measured and then installed. I suggest you use these values after a good draining of the old oil:  For the /5 use 100cc; later models 150 cc.  GS models don't use oil in the driveshaft.

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, no matter the oil level. The driveshaft level is not critical.  Some folks simply measure the oil and install the measured amount.  I usually do it that way.  It is also possible, sometimes, to use a tiny screwdriver or a very small allen wrench out of the on-bike tool kit a measuring stick, stuck in sideways, in the filler hole, which is located on the nose of the rear drive, but there can be errors if the driveshaft angle is flatter and so the most reliable method is measurement by volume.   If the oil is put into the filler hole too rapidly, it will bubble up and overflow. Drizzle it in slowly.

2.  If the transmission level has been lowering, it could, if rarely, be a faulty transmission output seal.  It can also, quite rarely, be the 13 mm hex-headed bolt located in the center of the top of the 5 speed transmission.  Use a sealant on the bolt threads and its washer.  I use Permatex non-hardening Form-a-Gasket.  Oil can be sucked up into the air cleaner housing and then to the carburetors if it gets by that 13 mm headed bolt.

If you are doing an input spline lubrication (often, erroneously, called a clutch spline lube), and see the input seal of the transmission leaking at all ...fix it NOW! is NOT difficult.  Otherwise, oil can move forward into the clutch area, due to that leaking faulty transmission input seal. Replacing the input seal is done carefully, without doing too much other than removing the transmission.  Drill a small hole into the metal part of the seal, 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.  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.  You do NOT have to remove the transmission to fix that. Tilt the bike to the right to avoid oil escaping, and remove and replace the seal, install to proper depth.

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) is 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, after curing, put a very tiny hole in it.  Later models don't have this as described.  The Paralever models have the output seal (no matter the color) put in REVERSED from the the other models.

3.  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's nearly always only an annoyance.  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 so the bike tilts slightly to the right, to avoid transmission oil flowing out of the transmission oil filler port when you unscrew its plug.  I have never seen, nor heard, of any Airhead having the driveshaft run near dry or to a dangerously low level from this oil transference to the transmission.  I am not worried about using the plastic syringe to transfer 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.   With a wedge in place at the left centerstand leg,  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 threads.  It is almost impossible to fix this cheaply.  Don't think about fancy venting ideas, they likely won't work.

Primary Causes for oil transfer from driveshaft 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 tiny 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 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 OK and is the typical amount recommended for all after the /5 (use 100cc for early /5, SWB).    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.  I recommend against that.   If you slide a small (tiny, such as a jewelers type) 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 more than plenty enough.

If the suspension has sacked out some, that way of measuring is not accurate.  So, one way to check the level is to drain thoroughly, insert the proper measured amount of oil, and then, if you want to, fashion your own measuring tool (thin and tiny, such as a jewelers screwdriver insert) 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.   Most of us measure the new-to-be-installed oil after reinstalling the drain plug.

h.  Combinations of the above items.

i.  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 & clear ...that is, not plugged.  Water can get into the transmission via that hollow bolt ...or ...via a bad speedometer cable boot (even rainfall with a faulty boot); 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 hollow bolt is 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, even forward.   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 two washers on this bolt; one is waverly; be sure you have them, otherwise you can force open that cut lug during tightening.  Put the battery lug between the flat washer and transmission or between the two washers.

j.  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.

k.  5-speed output shaft seal:

The R100GS uses the 'new style' output seal, and it, and the prior seal model, will look to you like it is installed backwards. The driveshaft on the GS is dry, it does not contain oil.   The notch at the 12:00 position just a bit above the output seal of the transmission, must be plugged on the GS.

If you install the output seal reversed, and you have a wet driveshaft model, the oil in the driveshaft will likely find its way to get into the transmission and raise the transmission oil level.

Installing speedometer cable rubber boot at the transmission output area:

Remove the hollow bolt (10 mm hex wrench) that holds the battery cable lug.  The hollow bolt is also is the breather for the driveshaft (and thereby the transmission on most Airheads).  The hollow bolt should have two flat washers, one each on both sides of the lug, and then a locking washer against the head of the bolt; preferably that is a waverly type washer. You can modify the battery cable lug by snipping it just barely enough to allow you to push it over the hollow bolt, which allows you to disconnect the battery without completely unscrewing the hollow bolt in the future.  By using the washers as noted, you will not spread the cut lug as you tighten the hollow bolt.  Never over-tighten that hollow bolt.

ANY of the below Methods work fine:

Method #1:  Push the rubber back on itself and push up onto the cable with the large end now upwards (pushed back on itself).  You will then be pulling the large end downward, so the small end stays upwards. Lubricate the cable and the top of the rubber (inside and outside) with a petroleum grease (not silicone grease, silicone grease is often called dielectric grease at the auto-parts store); after the rubber boot is installed and the large end pulled back downward.  Some use a drop of dish detergent, which will work fine.

Method #2:  Use a taper tool to expand the cable end of the boot, you could make a tapered tool, but you will find a caulking tube tapered sealing cap works fine. Use petroleum grease, as above, or drop of dish detergent.

Method #3:  Install a piece of shrink tubing; lubricate it with one drop of dish detergent, push the boot up.  You can do this without the tubing too.

Comments on those, or other methods:

Be sure you install the boot over the top lip of the adapter that goes into the transmission.  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 of water into the transmission oil is that the oil looks like creamed-coffee.  My suggestion is that you either put a fair amount of grease inside the boot; or, better, you seal the boot (my preferred method).  Sealing the boot is simple:

Push the small diameter top of the fully-installed brand-new boot down a bit, so as to expose more of the cable sheath.  Clean the sheath with a bit of acetone or MEK, or other fast drying solvent, using a clean rag.  The reason I use petroleum grease (using grease strictly for ease in installing the boot) is that silicone products are more difficult to clean off surfaces when you don't want remnants of silicone.   A piece of rag with acetone or MEK, etc., on the rag, will clean the small area easily, if petroleum grease is used. If you used a drop of detergent, clean it off with a water-dampened rag and then clean with acetone, MEK, etc.

With the top pushed-down some, and the cable and top junction well cleaned, put a small amount of black RTV sealant at the top, smooth it nicely with your finger, and then slide the top of the boot up a bit, and then reseal again slightly.  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 that point.

Some folks use a zip tie at the top area. That may work, but there can be problems,  especially so if the zip tie is a large one.  What happens is that tightening of the zip tie makes the top of the boot squeeze out-of-round, and this WILL 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 prolongs boot life, as the rubber compound BMW uses is adversely affected by smog and sunlight.


80W90 GL5 hypoid differential oil is what BMW specified for the rear drive, transmission, & driveshaft (non-Paralever) for Airheads.  Some who use synthetic oils find they have leaks.  If you use synthetic gear oils & have leaks, I suggest you switch back to petroleum oils, & 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.; although there are others who think the opposite.

You do not have to use 80W90. You can also use 75W90.

There are  wider-range oils such as 85W140, but I suggest that you DO NOT use them.... it is probably OK only for those in super hot weather and very high speeds, or pulling sidecars, or big loads, etc.  DO NOT willy-nilly change to 85W140.   I believe that it will not be at all any better unless the rear drive is exceedingly hot.  In MOST situations, use of these extra-wide-range gear lubricants is likely worse.  Do not use in the transmission, because such oils will change the spin-up/spin-down action of the various parts during shifting, besides not lubricating well under some situations.

04/21/2003:  Add .htm title; clarifications (minor).
02/04 and 06/2004:  minor clarifications.
03/06/2005:  Incorporate all changes & updates & fix typos.
01/21/2010:  Update during review of 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.
03/11/2016:  Update meta codes, layout, format, colors, fonts, etc.  Clarify details a bit.
09/27/2016:  Minor updates to metacodes, layout, format.
03/17/2018:  Reduce html, colors, fonts.  Improve layout.  Add 10pxl margins.
10/11/2020:  Update

Copyright 2020, R. Fleischer

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Last check/edit: Sunday, December 06, 2020