02 rm125, hole in piston

I just rebuilt my buddies 02 RM125 with a new wiseco piston. Cylinder was replated also. We went riding at a sandy track, I hadn't been on a 125 in a while so he let me take it for a spin. I wasn't afraid to get on it as he had broken it in days before. Theres a huge deep sand hill and I come into it 4th tapped. It starts to bog so I clutch a litte, still bogging, 3rd...then bbbbwwaaaaaaaaaa, thats all she wrote. As I'm rolling back down the hill, in gear, and its turning over like nothing I knew exactly what happened. Hole in piston, and burned the electrode off the plug.

So....I've got new piston on order but I'm kinda p/o on why it happened in the first place. I had been doing plug checks before I really got on it and that looked good. Was running piss 10% ethenol 91 pump fuel at 32:1 amsoil. What are some things I should look for so this doesnt happen again?? Well, so he wont do it because now I refuse to jump on that bike again :bang: