may be the carbon buildup would be there try scrubing the carbon deposiuts with a shaving razor blade,,use gillette double edge shaving blade,,
it will come out flat if it is or it will remain doom,ed shape,,
if ur new piston is flat and is of the same height as is at the centre of ur old then u r getting more compression,which will result in detonation and preignition due to carbon buildup,
if ur new piston is the same height as upto the lowest edge of old u r not getting reasonable compression ..advantage is u can have no issue with poor quality fuel,disadv is slightly poor performance,,
verify u r using correct piston,,
