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Message: [QUOTE="Magellan, post: 1433444, member: 22253"] I recently rebuilt the top end on my son's KDX 80. I had the cylinder re-plated and put in a new piston and rings. I got the piston from the company that did the cylinder repair (welded up a pit, bored to match piston and plated). I never checked the clearances other than the ring clearance. The engine has about 10 hours on it and has been running fine...till yesterday. My son was riding on a forest road to get to the trail head. He is just learning to use a clutch, so was probably running a pretty steady speed in first gear for a while. We were in a bit of a rush, so we didn't warm the engine up for very long. About a mile up the road, the bike lost power. Seemed to me like it seized. The engine didn't lock up, but there was no power. It would start, but didn't run well. I tore down the top end today. The cylinder and piston look fine. Some light scoring, only one that barely catches a nail. No signs of metal transfer and everything measures within spec. A compression test prior to the tear down showed 120 psi, which is within the service limit. I did notice some signs of overheating on the flywheel near the connecting rod. Could the bottom end have seized? I was running a 42:1 mixture. The manual calls for 32:1, so technically this is lean. Could the bottom end overheating cause a loss of power without actually freezing up the engine? The connecting rod has a bit of radial play. To the extent I am able to measure it without disassembling the bottom end, it is probably out of spec. I don't think it is any worse than when I did the top end though. There was a lot of piston slap when I bought the bike, so I just did the top end. At this point, I'm thinking of just cleaning up the piston a bit and running it with a richer mixture. Here's a link some pics of the piston, cylinder, and flywheel: [URL=https://www.dropbox.com/sc/v1v4jkey5trqkqt/JTob-O_Uys]KDX Top End[/URL] Any input on what you think happened and what I should do are appreciated. Thanks. [/QUOTE]

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