When you remove your flywheel (called a Clutch Carrier in 1981 and later models) for such as to replace a mainseal, oil pump seal..etc., it is CRITICAL that the crankshaft be BLOCKED from moving forward.  If you do NOT block the crankshaft, you run the risk of the crankshaft moving forward a small amount. This can result in a HIDDEN thrust washer moving downwards and off its two locating pegs. If that happens, the washer will not align back on those two pegs as you tighten the 5 flywheel (or clutch carrier) bolts, and you run the risk of MAJOR DAMAGE to the thrust washer….and even MAJOR damage to the engine casting.   The crankshaft will begin to freeze up as you tighten those 5 bolts. There is another similar thrust washer located rearward of the engine casting, and it can also come off its pegs, but you can see it when the flywheel is removed.  I install that rear thrust washer oiled, which acts like mild glue when installing the flywheel (clutch carrier). You REALLY do NOT want any of these thrust washer movement of the pegs problems happening. If you forgot or did not know about blocking the crankshaft, and you want to know IF the crankshaft has moved, there is a link near the end of this article, which will give you the information. The dimensions need to be as shown, in order to know that the crankshaft has not moved. The only way to see and get the hidden thrust washer back in place, if it has moved off the pegs, is via removal of the right cylinder…..or MAJOR disassembly of the engine!   The second thrust washer I mentioned is located at the rear side of the rear engine boss area, and as noted, you can see it if the flywheel (or clutch carrier) is removed.  Be sure when assembling the flywheel (or clutch carrier, as it is called from 1981), that it is on its two pegs, note what I said about some oil to make it stick nicely, so do that.

Blocking the crankshaft is actually quite simple, and is usually done by some sort of makeshift tool that places a bit of pressure against the alternator rotor center bolt. This can be done in many ways, even a piece of lumber such as a 2 x 4, bungeed to the cylinders and with a screw to press against the rotor bolt. A small easy-to-make tool can be made of a piece of allen wrench with a welded or brazed fender washer on one end, over-all about 3/4 inch. The outer cover holds it in place with a slight amount of pressure from the cover screws. A picture of this sort of small tool and its use is on Snowbum’s website, on the link provided at the end of this article.  It is also shown in item #8 of the following article:
https://bmwmotorcycletech.info/tools.htm

Another type of tool, no need to make a welded washer and allen wrench, is to just unscrew the allen bolt in the alternator rotor, and remove it. Replace by a similar threaded bolt, but much shorter.  Adjust to proper length.

A sketch of the flywheel and crankshaft of your airhead, with additional notes, is located here:
https://bmwmotorcycletech.info/flywheelremovalwarning.htm

Snowbum
copyrighted 02/19/2021
https://bmwmotorcycletech.info/index.html