I see a lot of misinformation on the oil in the engine, transmission, swing arm
(drive shaft 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.
BMW motorcycle 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 swing arm (area of the boot) is much higher than that in the transmission
when you are riding.
The swing arm oil, at the proper amount, can't get into the transmission unless
the seal is bad, or 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 if 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
It is quite possible, even common, for the oil to flow out of the filler hole
when the plug is removed. I'll bet that the color is sort of gray and
frothy. Water has gotten into the transmission. That is partially
why the level is too high. The other reason is that it gets mixed up with
air. The water and air make for a much larger volume that is then forced
out of the filler hole.
The water gets in via the speedometer cable. Have you been riding in
the rain? The rubber boot that is supposed to cover it has a leak or isn't
seated properly. There are many schemes to assist a perfect boot to
further resist the water intrusion. Some fill the boot with grease and
some seal it with Silicone glue.
By far the largest problem is the water in where the bearings live. If
it is a short time then maybe the bearings will be OK. Drain the fluid and
refill to the bottom of the threads. Ride it for enough miles to get it
good and warm. Drain it into some container for close inspection.
You should see no contaminants of any kind. Keep refilling and draining
till it comes out clear.
Swing arm (drive shaft 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 drive shaft 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 maybe 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
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
Final drive (rear end)
A common problem with the /5 (to some extent the /2) is that some of the oil from the drive shaft
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 path(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
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. Hylomar HPF.
Available at auto supply stores with the rest of the permatex products.
The permatex part number is 25249.
The three paths for oil leaking into the final
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 of the ten studs. We just filled it
with goop. "Goop" is your favorite non-hardening sealer. Some of the
covers and cases had porous castings. Does someone have a picture of one
of the 1970 mis-machined covers?
The largest engine oil problem was very simple, no oil. The next
largest problem was infrequent oil changes. What oil is best? I
don't go there. Don't go to any of the lists and ask either. This
argument goes on forever. I have no doubt that some brand or type of oil
is better than another, but I don't have that info, as I only have my personal
opinion. It is not based on scientific evidence.