Took the engine out of my zip yesterday to replace a final drive bearing and figured while it was out of the bike I could split it and fit a much needed uprated crank. I noticed the piston rocked a bit side to side to took it off too and found this: Small end bearing had seized in the conrod and machined a lovely cutout into the gudgeon pin. Unfortunately the pin has scored up the piston where it fits through. It does explain why the engine was running a bit hot on a ride last week. Weirdly managed to cover 300miles like that with no visible damage to the barrel and no power loss and only symptom being a metallic grinding noise! under load. Still need to measure the barrel thoroughly to make sure its still round.This was on a stage 6 sport pro cylinder and pro rep exhaust on an original crank. Kit was fitted with 500miles on the crank and covered 1100more since. Lots of lubrication and mixture spot on so think it was just a case of the small end bearing not being up to running at 10k+ for prolonged periods of time and a bit of bad luck on my part.
