Did you check the crank and rod for any slop before putting the piston in? if the rod has any play, or the crank bearing is worn, it will slap that piston around the cylinder as it flops around. Sounds like clearance problems, especially with the back side of the piston taking the damage, like there is slop in the power train allowing the piston/rod/crank to float to the back under the power stroke.

 

Everytime i bought the piston and rings from the shop that bored or honed it(same shop everytime). The last time i took crank and whole bottom end with rod and they said all were well within spec. Is there a shop that's good? How hard is it to spec the crank? The last time i helped put it together and everything seemed real tight but heck everytime did..