Oil transfer into the final drive.

By Duane Ausherman

   I see a lot of misinformation on the oil in the transmission, swing arm (driveshaft housing) and final drive, so this may sort some of it out.  The procedures for filling with oil are shop derived for accuracy and speed.  Ignore them if you want to spend the whole day doing a 1/2 hour job.  

Transmission (Tranny)

   The /5 transmission requires hypoid oil and latest reports are that the newer and more expensive synthetic oils help the transmission last longer before needing a rebuild.  The rear seal is to keep oil that is in the swing arm from getting into the transmission, not the other way around.  The oil level in the area of the boot is much higher than that in the transmission.  The swing arm oil, at the proper level, can't get into the transmission unless the seal is mounted backwards.  It has happened before. 
   The proper level in the transmission is just to the bottom of the filler hole threads.  It is about 800 cc.  It is better to have the oil level a bit low than a bit high.  It would work just fine if it was 3/4" low, but I wouldn't recommend it.  It is best to always use the same level and then you know of some change has occurred that needs attention.
   One area to watch is the "shelf" under the transmission and at the rear of the engine.  It tends to collect oil.  That oil could come from the transmission input shaft seal,  engine rear main seal or both.  Hypoid oil smells bad so you can diagnose the source of "leak" from the smell.  On a /2, the leakage of the input shaft seal could indicate a bad rear main engine bearing.  On the /5, and later, that leak may only indicate a bad transmission input seal, bearing or both.  A bad bearing will allow the shaft to move around and the seal may actually be good, but it can't seal up a "moving" shaft.

Swing arm (driveshaft housing)

   The swing arm also uses hypoid oil and it is a measured amount.  The swing arm does not have a reliable "level" that can be checked easily.  It is often said to fill it to just cover the driveshaft while peering down the filler hole.  That amount is fine for lube purposes.  Depending upon how the bike is parked, the amount will vary and that isn't good.  Use a measuring cup to know how much to add to the swing arm.  Follow your owners manual.  The amount should be in the area of 100-150 cc.  If you aren't sure, then use the 100 cc figure.  It is far better to have less oil than to have too much oil.  The bike could work very well for many years with only 50 cc in it.  Many times I have found the swing arm empty of oil.  We added oil and changed it after some miles.  Usually the U-joint wasn't damaged, but the owners were lucky.
   Here is a very easy way to add the oil.  It is faster and more accurate than any other method that I know of. Buy a plastic funnel that has just the right size small end.  It should sort of "screw" into the threads of the filler hole.  That will seal off the air.  Pour the oil into the funnel and it will just sit there, due to the air lock.  Squeeze the rubber boot.  That will force air out of the swing arm and you will observe bubbles in the oil.  When you release the boot, it will expand and suck the oil into the swing arm.  Repeat a few times as necessary.  Since you are adding a measured amount, you need not waste time trying to observe any level.

    Always drain the oil from the drive shaft (swing arm) into a measured beaker and record the amount in your journal.  This will become important over time.  The only three places that the oil can go is into the final drive, the transmission (if the seal is in backwards) or leak out onto the ground.  Most leaks will be visible at the drain plug or the boot.  The drain and filler plugs are prone to getting striped out in the /5 and later.  Tighten them only very little.  It only holds the oil in, nothing more.  A crush ring can be reused if it still looks good.  We replaced them in the shop only because of liability.  On our own machines we seldom replaced them.

Final drive (rear end)

   A common problem with the /5 is that some of the oil from the driveshaft housing leaks into the final drive. The indication of this problem is that when one removes the final drive filler plug, oil runs out. Don't panic, there is no big expensive emergency here.  Drain out the excess and add it back to the swing arm.  Don't obsess over the possible shortage of oil in the swing arm, go back and read the above.  When adding oil to the final drive it should only come up to the bottom thread.  It takes too much time to keep looking at the level and adding a bit more and then it gets over filled and must be removed.  Measure the oil in a beaker and dump it in.  While it is running in, you can do some other job.  
   You will finally get tired of transferring the oil and want it repaired properly.  To do so you must find, and correct, the paths(s) that the oil follows to get into the final drive. There are 3 of them. Any BMW mechanic should know that.  You should question your prospective mechanic and if he doesn't know that, hit the road. Take it to a good mechanic.  You may have to ship it out and wait for it.  Don't trust just anyone for this job.

   You can remove the final drive and take it in without the bike. The final drive doesn't need to have the cover taken off for this job. I would remove the cover anyway because the ultimate life of the final drive is the gear wear. I would want to inspect the gear wear pattern and correct the spacing if it is wrong, before you must buy the gears too.  See my page on how to inspect the final drive.

   Another reason for removing the cover is that the brakes would sometimes get wet with oil. The drain hole didn't help here.  The drain hole only drains oil that leaks past the seal. Check the drain hole yearly, with a wire, to see that it is clear. It is located just below the axle nut.

   The higher oil level, caused by transfer, causes excessive pressure in the final drive. Then the oil leaked along the threads of the 10 studs that hold the big round cover plate. This is why the oil got onto the brakes, and was a safety issue. This was a frequent occurrence in 70-71.

   The big nut holding the splined gear must be removed using a special tool. The threaded ring that holds the seal must be removed using a special tool. Clean everything totally.  Do not use Silicone.  We used Hylamar, but I hear that many modern types of goop are good.

The three paths for oil leaking into the final drive are:

1. Past the seal, in the old days this was the least likely path. Replace the seal.

2. Down the splines on the inside of the gear. Goop them up.

3. Along the threads of the seal holder. Goop them up.

   When these bikes were new, they often transferred oil from the swing arm to the final drive badly. We could fix one without replacing any parts. The parts weren't bad, the assembly of the parts was less than perfect. Isn't that diplomatic?

   The early models in 1970 had miss-machined covers. A recessed groove was allowing oil to get to the threads. We just filled it with goop.  Some of the covers and cases had porous castings.

Edited 4 September 2002  Please email me about confusion, errors or typos noticed.  w6rec@softcom.net