Oil Transfers....oil moving someplace other than where it belongs
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oiltransfers.htm
article 49


1. If the REAR DRIVE has more..OR LESS... oil than it should have, and the driveshaft oil level is lowering, 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...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, and if it loosens, bad mechanical things happen.  

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.

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, etc., will throw the very cold oil upwards and tend to plug the breather; or, moisture has 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.  CHECK the rear
         drive oil level now and then.   Normal oil level is at the bottom of the threads of the filler plug on very early models.  On later models, there is an oil plug
         (extreme rear, centered, small plug).  That plug must not be over-tightened.  Proper oil level is such that the oil barely runs out, if plug is removed.

 I think that 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 have done it that way sometimes, preferring to use a tiny screwdriver or a small allen wrench out of the on-bike tool kit as a measuring stick. Keep in mind what I said about trying to measure the quantity, if the driveshaft angle is flatter.    BTW, if the oil is put into the filler hole too rapidly, it will bubble up and overflow. Drizzle it in slowly. 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 transferring to the transmission. 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 three.

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 the washer.  I use Permatex non-hardening Form-a-Gasket.  Oil is being
    sucked up into 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. 

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. 

 NOTE 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 a very tiny hole put in it.  Later models don't have it.    NOTE what I said about the greeen 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 longer rides. What you can do is to put a thin wedge-shaped piece of wood or plastic under the left leg of the center stand...which cants the bike slightly to the right.  I use a plastic wedge that was intended to help install tire chains.  This wedge is for preventing oil from flowing out onto the ground.  Unscrew the transmission filler/inspection plug, and remove some oil, I use a very large 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 to the driveshaft; remove the wedge and tilt the bike back slowly, to avoid oil running out.... recheck transmission oil level. Repeat until level is just at bottom of 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:
1. Sagged suspension.
2. 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. 
3. 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.  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.
4. Considerable steep downhill riding, like in the mountains.
5. Heavy load...like 2-up or heavy rider or both
6. Very bumpy roads
7.  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 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 fashion your own measuring tool, for YOUR bike.   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.  
8. Combinations of the above 7 items.
9.  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.  This is the ONLY breather for the transmission 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 a pair of washers on this bolt, one is flat, one is waverly; be sure you have them, and put the battery lug between them.   NEVER allow the rubber boot over the speedometer cable to get old and fail....it is a very good idea to clean the top area and junction and seal that black rubber boot to the speedometer cable with black RTV.  Some will pack the inside of the boot with grease.  I don't.  NEVER spray water at this speedometer bolt and boot area when washing the bike.  Water that gets into the transmission via that hollow bolt...or...a bad speedometer cable boot (even rainfall does that) or nasty spraying during washing and it will  destroy the transmission bearings!  Replacing that boot is best done with a tapered tool, you can make one from such as a caulking tube sealing top.   The typical leaky place is the actual top of the rubber boot, where the cable rises upwards....the top expands.  That is why I use RTV at that point.  Clean the cable at that point with acetone, so the RTV sticks really well.
10. 80W90 GL5 hypoid differential oil, NON-synthetic, is what BMW specifies...and the SAME for rear drive, transmission, driveshaft (non-Paralever).  SOME who use synthetic oils find they have leaks.  IF you have that problem, switch back to petroleum oils, and the seals may fix themselves, after considerable number of miles.  You can also use 75W90, or wider-range 85W140 (probably better for those in super hot weather and high speeds, etc).  

 

Revisions:
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

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