If you recall, when I drained the oil out of the final drive I found a bits on the magnetic drain and a chunk of metal that was a wee bit concerning:
After reading up on airhead final drives, I realized I could remove the inner cover without disturbing the gear setup. (That anti-seize lube crap is a bitch to clean out of the splines!)
Well I found the rest of it and I was able to put Humpty Dumpty back together again - sorta. I couldn't find a parts diagram for the final drive, so I have no idea what this is, but it looks like it's supposed be a trust washer that is located at the end of the needle bearing in the center of the drive.
You can see here there is metal smeared on the shoulder of the flange and there is some blueing from overheating visible too.
So this drive is toast. Too bad as the bevel gears actually look very good. This was a 32/11 drive with a ratio of 2.91. It's one of the tallest rears BMW made (I guess the R100S was supposed to be a powerful long range highway cruiser.) I've decided to replace it with a used 32/10 (3.20 ratio) unit off a '73 I found on eBay. This will screw up the speedo drive ratio, but who cares? Bonus is the brake pads are included - I hope they're good enough to use.