It's simpler to measure the pistons- - - -a 3 9/16" bore .030" oversize will be 3.5925" The pistons should be a couple of thousandths smaller than that dimension, as the clearance is built into the piston skirt. Measure the piston at the height of the wrist pin, and 90 degrees away from the pin boss. Pistons aren't round- - - -they get smaller as the measurement moves away from that 90 degree area, and the piston crown where the rings fit is smaller than the skirt by several thousandths. If the engine has some mileage on it since the last rebuild, it might be necessary to run a knurl on the skirt in a couple of places to get the clearance back within specs. I like to run somewhere around .0025"-.003" clearance on a cast piston. As the piston heats up, the clearance tightens up a little, and the cam-ground shape gets round because the Invar strut across the wrist pin area won't let the skirt expand much.
