Took my motor apart for a slight rattle noise, plus the fact it was changing 2 gears at once sometimes, and hitting neutrals occasionally. LH side (cam chain) main bearing had 2 thou play in it, but also I found this.. Now, the piston is a new CP piston (98mm), the rod (aftermarket, Willpower rod) had 30 minutes on it when I put the new piston in (I bought the engine off a Supermono racer), and I had no reason to believe there was excessive play wwhen I put the pin in. The piston is not hitting the head, but on the exhaust side there is no carbon build up, so its very close there, but is deffo not hitting, so I cant see it whacking the hell out of the little end, although if it was, it is on the right face of the little end to do so. I have half a meter of phospher bronze bar, so I can easily make another (my old LC4 KTM supermono racer had one in that I made for two seasons), so I can fix it, so its not gonna cost me anything to fix. plus at the same time, I will take a few thou of the piston crown on the ex side. The little end free play (side to side waggle) is 0.8mm (standard tolerence), there is no big end play, so the rest of the rod is OK. I use Motul Factory Line 300V 10/40 oil, and change it after 2 meetings. I use the bike for supermoto. Any ideas why this would happen?