there used to be slightly a .001" oversize of std pistons available.measure the cylinder in several places first! then they went to .020',.030",.040" but i dought you would be able to find any now some N.O.S. stuff still might be around somewhere.sounds like a tapered bore there.very common. they do have offset wristpins so they have to go in a certain direction. offset is 5/64" and you do measure them at the center of the wristpin. they are cam ground.meaning not round when at 72 degrees. piston to cylinder clearance at min is .0005"to .0011" that is with stock pistons only! they have 2 compression rings at the top and 1 oil ring on the lower ring slot.the comp rings are hastings and are cast iron the top ring is usually chrome plated. the oil rings are thin cast iron with a spring expander to keep them apart install with gaps 120 degrees apart.oil the rings and the ring grooves with engine oil.ring gap is min.007" to.020'on the comp rings and .005: to ..015"on the oil rings on a thriftmaster and .010" to .018" on the loadmaster oil rings. as a rule figure 003" gap for every inch of bore for the comp rings. this motor has a std bore of 3.620" or 3 9/16". so.003" x 3.620= .010"the compression ring are a reverse twist type the face of the ring is usually cut on a slight angle follow the new ring instructions in the ring set for installation as marking may vary from a dot meaning that side up most cast rings have no marks on them moly style rings are not usually available for this motor but total seal does make ring sets p/n 6310 but are really not nessessary for a street motor.enjoy the repair.
