[PC/SR]

Where are you measuring? As noted by Kirk, the top is smaller than the widest part of the bottom of the skirt. A 3.875(standasrd) bore should have a 3.872 piston diameter measured at the widest part. Measure the bottom of the skirt 90 degrees from the direction of the pin. That should be a few thous less than the bore. Also measure the bottom of the skirt in at the same direction as the pin. That should be .005 or so less.(This is called cam ground, ie, not perfectly round.) If the piston is the same diameter all the way around there is going to be trouble unless you have really large wall clearance. A buddy's engine froze up, tore it down the pistons were all scored, and were the same diameter all the way around.