I would like to see your OS engine-s thats has peeled a nickel liner or chrome ,gives us all the particuars ,plug type ,fuel used ,prop size and RPM and of course engine size .Also would like to know what the manufacture did to resolve the issue with said engine or engines .The only reason I have seen that makes a cyl or piston score in the exhaust port is due to over heating or lack of oil .Dirt is a possible but as a rule it wont pick just the exhaust port its all around the piston .