I have a 5 STar I just rebuilt. M5 jugs bored to 4.8" with Ross pistons and new rings that came through Ross. Engine ran ratty so I checked and found #3 & 4 dead. Compression was 0. #1 & 2 were 240.Expecting broken rings or burned pistons, we removed the pistons and found everything good except we could see where the valve impressions could barely be seen on the piston surface. The rear of #3 piston was worn at the top with obvious scraping and a little scarring on the cylinder. We couldn't see anything wrong with #4 piston or cylinder. Does anyone have an idea what might be my problem. Thanks for any help.