Thanks jwb - we've found a few spots where the manual has had us stumped... this is really helpful! Thanks Ladder106 - The cylinder still measured to spec and the piston looked really good, but we weren't confident with our micrometer so we brought the cylinders and pistons to the local machine shop here. They offered to run everything through a parts cleaner while it was there, so we're heading back for an official diagnosis and hopefully to pick everything up in a few hours. We still need to look at the valves. Fingers crossed.