Possible that you had the wrong piston in the wrong hole?

I built a motor like that once... Accidentally swapped piston number 4 and 6. When I checked piston to valve on #1, it had plenty. When I built the motor and switched the piston, the exahust notch (eye brow) didn't have enough clearance for the intake valve.

Idled fine... failed under power after a little while from contact at higher RPM.

I wonder if you did the same thing? Dropped the valve, and killed the piston?

Good luck... sorry to hear about your troubles. Hope it isn't too expensive or time consuming to get it back together.