Depends on the piston type, cylinder clearance, intended rpm and rod type and quality. I'm running .035" on a small block with KB hypereutectic pistons and about .0025" clearance on the skirts. Runs great. I wouldn't be afraid of .030" on that one either. But you start opening up the bore and using forged pistons, you'll want more clearance because of piston rock and growth. I suggest shooting for zero deck and running the quench at whatever your gasket thickness is. On a Fel-Pro it'll typically be .039".