There are long feeler gauges for this . Place the piston half way or more down the bore , not up top , a 4 thou . strip down between the skirt and the bore on the front or rear thrust face , not inline with the piston pin bore . If its a fairly firm tug to slide the strip out with a 4 it's usable . Tighter can often nip up , looser will give rise to slap . Many go much closer , down to 2 but it's very prone to pick up or seize in the bore if not broken in VERY gently . Not for us . My thoughts anyway . Sid .