Your ring gap is exponentially proportional to the bore size. Even an increase/decrease of just .001" can change the gap significantly. If the piston to cylinder wall clearance is at or near the max, so will the ring gap. From what you mentioned was stamped on top of your piston, that would surely indicate the overbore size of the engine, so if your trying to install standard bore size rings in it, that is your problem. To remove all doubt, just mic the piston to verify its size.
