Tom,

it could be the small end bearing, or the big end bearing.

For the piston, you shouldn't have any play at all on the small end. Side to side maybe, but nothing else. The big end will have some play, and that is outlined some where on NSR-WORLD (I am sure, just don't know where off the top of my head). But again, I think its mostly side to side play. If you got slop in the crank, then you will need to replace the entire crank.

Thanks,
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Charles Gallant