WHat was original piston-bore clearance?

How was the engine run in, and for how many miles?

Forged pistons expand quite a bit under high boost applications, and also as they are aly, wear easily...could be a couple of reasons, if it had picked up a piston it would be fairly obvious depending on the severity, are the sides of the bore polished looking or scored? probably wise to remove a piston, check for wear and check piston/bore clearance is still within spec.