Does anyone have any idea what might cause this level of damage to a piston in normal use? Its from a DZ160 engine I sold to a chap 6 months ago and he's just sent me this picture claiming that its been like this since I sold it him. Yet the engine ran fine last time I used in in Argentina 2007; it had to 'cos of the howling winds there. I can't believe the engine would have run if the piston was like this.

Keith