From the Airheads Mailing List
>>Oh, no! I noticed oil running (drooling?) out behind the trans. If my diagnosis is correct, what seal is it?
I presume that when you say "behind" the trans, you mean at the trans-case interface? If you mean literally "behind" it, as in out from the boot at the trans output, you can stop reading. If not, you more than likely have a rear main seal failure. That's the seal where the crankshaft emerges from the case to marry the flywheel.
Pitfalls to avoid?
Other items to replace or rehab at the same time?
With the bike now up on an elevated table (we call it the "surgery table") and with the final drive, etal all the way off, it would be a shame to take it to the dealer. Plus the $$$$$ consideration.
Then this is the IDEAL time to do it. You have to pull off the final drive to pull the trans anyway, and that is a big part of the labor. You will need a few special tools... a flywheel holder (holds it in place so you can unbolt and later bolt the flywheel to the crank), a set of three ( I think) longish metric *fine* bolts for backing off the clutch's diaphragm spring, and finally a tool for seating the new seal to the correct depth.
Some on this list have suggested workarounds for some of these tools, and some will also say that you need a clutch alignment tool. As always, YMMV. But I recommend that you buy the tools NOT from BMW (expen$ive), but from Ed Korn's catalog of BMW tools. All together they will run you about $45-50 as I recall. All of his tools (which others can confirm are of high quality and intelligent design) also come with instructions, though they are almost self-explanatory. You will also need to get some new flywheel and clutch bolts.
Pitfalls... assure that you prevent the crank from moving forward... You can use a spacer on the alternator side... this is also explained in Ed's fine tips.
Tips? Give yourself plenty of time.
Other items to replace? Well, you might as well give the clutch a good inspection. You'll have to take it apart to get to the seal. You'll probably also find that the clutch "housing" will be caked with which I assume to be a mix of friction plate dust and the oil you've been blowing by the seal...
And of course, there's no better time for a spline lube!
My experience (confirmed by others) is that an alignment tool is unnecessary. Just *gradually* tighten the clutch while sticking your fingers through to assure that the splines remain centered. The friction disk rides on the trans input shaft, so if the trans slides back on it is by definition centered. Just work the clutch a couple of times to get things right.
NB that is is possible that the root of the problem is not the seal but a crankcase breather that is not releasing pressure. So, you might as well call the dealer and get a new-style reed-type breather and do it too.
Of course, if your oil is NOT coming from the side of the trans that I assume, you can safely disregard everything above.
Thought I did this LAST winter and thus have a faded memory about the project, I'd be happy to try to answer any questions you run into. It's a long job, but with the final drive off you've already done some of the work. And in any case it beats the hell out of paying a dealer to do what is really a labor intensive job.
Best of luck...