No pics of the barrels im afraid. This was the first sieze Andy and i did have squish and compression problems, but they have matched the heads re honed the bottom barrle, replated the top and new pistons and rings.

This time its gone pop and i dont know whats caused it yet, Wayne thinks its may be a coller/bush that sits inside the barrel is missing, I can only see a bush part on the ns250rg and mc18 parts manuel, Do you have a part number for the 21?

Or like fontyy said in a previous post it could be another bit of ring that hasnt been flushed out properly???

gota bear in mind that the pics posted are the pistons that have been in the bike for good knows hows long, You did the crank did you change any rings/pistons etc, the only other engine work in this area was the head conversion which i fitted the wrong head, but that was the frnt barrel not the top!!! which is causing the problems!

What else could cause the marks on the piston? bad powervalue set up, worn bushes?