I like the way FPE does it. Their engines ship with an aluminum prop washer that is drilled either 5/8 or 8mm (they are close) but the washer has a small, maybe 1/8 inch protrusion or *** that is on the side that goes toward the engine. This protrusion is exactly 10mm outside diameter and that takes up the slack. Practically all the larger props are drilled 10mm.

What FPE does makes so much sense I don't see why all engine mfgs don't do the same thing. The Brillelli 46cc I recently bought came with an aluminum bushing, which is ok, but not as good as FPE.