Not sure what the problem is! Definitely not normal having to replace the piston/liner after 6 or 7 flights. Who made that determination, you or the salesman? Do you fly where there is alot of dirt on the runway? One of the first things lots of flyers do to a new engine is remove the backplate and inspect for any metal shavings from the machining process. Although not so much anymore with the newer technologies in machine equipment, it can still happen. Doesn't take much to score a liner.
How is the engine running? Can you tune and fly it at all_bob