I just took apart my merc inline six head, removed pistons, one (#5) was bad and one (#3)was scored on side, all had 120 compression except #5 which had no compression before tear down. I plan on replacing all pistons and rings but do I have to rebore all cylinders? The cylinder wall on 5 and 3 (number three has next to zero mostly on the piston) have scratch marks that aren’t badly gouged but you can feel them running your fingers over them. How bad do the cylinder walls have to be before reboring?