have u removed headshims from the engine? make sure there is atleast 1 head shim between the head and the sleeve because the glowplug may be abit too close to the piston or u dont have a the brass washer on ur glowplug....come to think of it my navorossi .12 has got a totaly black piston the top of the piston is black like a permanent marker...i got the engine 2nd hand and its not pullstart so i cant realy start it up