Fork seal ring for the bottom of the fork on the /5 series BMW motorcycles. When loosening, or tightening this big nut at the bottom of the forks, one must not do this procedure with the forks held at the fork stop. There are a variety of ways to do this safely. A simple one is to use the axle, just insert it into the axle hole and put a pipe on it to extend it for more leverage. Now, all forces are on that fork leg and not one the fork as a whole. That is a great threat to the fork alignment.
This Service Bulletin is listed lower on the page, but just in case that you don’t scroll down that far, I will list it here too.