Ooops - you're right about the typo, Ron. Many apologies. 15/64 is what it should be - and I think the extra clearance would be important, too, from the sound of your experience with your Triumph..
I'm going with 1/4 by 1/4 rollers for my wm20. Just have to wait for them to arrive.
Without trying to sound like an expert I'm sure 1/4 x 1/4 are correct for M20 clutches. 1/4 x 15/64 are the size used in Burman clutches on Matchless etc.
Richard
