I have now measured the 2 ball bearings that came with the timing cover. One is 1/4 inch and the other is 3/16 inch. I still can't figure out for sure which one goes under the spring as either will fit and whether or not there is a place for the other one or it just happened to be in the timing cover when I got it. Confusingly some parts lists seem to list both bearings but I can only see one in any parts diagram of the timing side - the one under the spring which I think is the small one. Anyone out there recently had their timing cover off and could advise? Thanks, Mike.
The 1/4" ball bearing fits in the cover under the spring...The other one just came along for the ride and isn't for an M20...
Fit the ball into the cover and then give it a light tap with a hammer and suitable punch to 'seat' it into the cover...Then refit the spring and retaining fitting...Ian