Hi Mark..Crank bearings were all C3 fit originally. However, some of them are getting increasingly hard to find in that fit (particularly the Imperial ones) and very expensive. Most crankcases now have some wear in the bearing housings and I have found standard fit bearings run OK.Saying that though, the 6205 drive side ball is easy to get in C3 so you might as well use what you can readily get.
No problems running a concentric on an M20 but remember when selecting jets to look up the Monobloc settings for the m20, not the 276 ones. Monobloc and Concentric jets are the same. A 930 is a bit big for the M20 and it may have the wrong slide cutaway if it came off a B50...but you should be able to get it running ok with a bit of fiddling...Ian
email (option): ian@wright52.plus.com