Otherwise known as the differential output shaft seal.
This shows how to do a quick on-the-bike change on a BMW R1100 or R1150.
Note that this job can be done in less than an hour (including wheel removal and replacement) with nothing but the tools in the stock under-seat kit of a BMW.
I’d just come back from a thousand mile trip and started to remove the rear wheel to swap it with another bike (managing tire wear, doncha know), when I noticed grease dripping from the final drive onto the wheel and running onto the tire.
I checked the side-to-side and top-to-bottom play in the tire and found only a minute amount, which gave me hope that the bearing assembly wasn’t failing. So I bought a replacement seal at my local dealer (A&S $30.97 ) and set about to install it.
First, put the bike on the center stand and strap the stand to the front wheel.
Then remove the rear wheel. It helps to have someone step on the brake while you loosen the lug bolts, so remove the brake caliper after the lugs are loosened.
If you’re curious to see how much grease you lost, pull the fill plug and check with a clean instrument. Mine was only down a little.
Next drain the remaining grease and check for bits of metal on the drain plug and in the oil grease. Mine looked normal with just the usual fine “dust.”
Next take an old screwdriver or some other blunt blade, and slip it between the output shaft (where the lug bolt holes are) and the seal and gently pry it out. It doesn’t take much force.
Be sure not to score the final drive housing. It’s aluminum, and it would be easy to score, which might affect how well the new seal would seat.
Take a rag with some solvent on it (I used gas) and wipe around the output shaft to rub off the “lacquer” that builds up on it.
Put a thin film of clean grease on the housing and the output shaft to help the seal slide in and seat. Final drive grease would probably work just as well.
Slip the seal into place and gently press it to with your thumbs, being careful not to push it too far in. You want it flush with the housing.
Fill the final drive with grease.
Replace the rear tire and brake caliper, and you’re done.