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Melted piston

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I have a 96 yz 125 that I made a big bore 137 and I rebuilt it about 6 hours ago and I run it hard but not like reved to the max and I rode it down my road fast and the bike locked up and then it kicked it over and it would not run and I tore it down and the head has metal that cooled down on the head and the rest of it is fine but above the exhaust port the cylinder is a little ruff but the piston melted in the middle it did lock up like this before like at 5.5 hours and I just rode it to the base camp but it had low compression any ideas what happend I run 45:1 amsoil dominator oil and 93 octane and can you hone a plated cylinder to make it smooth

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With that much heat and 45:1 It will probably need to be bored to 144cc and replated. If you decide to rebuild it do yourself a favor and run a minimum of 32:1 in the bike. If I was going to be making road runs on a 125 I'd probably even go to 28:1 or 24:1. Too much oil won't kill it but you see the results with too little oil.........

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Too little oil will not cause a hole in the piston.

That is from too much heat caused by lean jetting, air leak , incorrect timing or shitty fuel.

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The gas I think was old because I have never had a problem before but I wanted to sell it before this happened and I put some gas from November of last year but this happend months ago

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Did the head get milled to accommodate the increase in displacement?  If you just upped the bore and put the stock head on it you substantially raised the compression ratio which could cause this kind of detonation.

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Detonation will not cause a hole in your piston.

Check your timing, compression and do a leakdown before running it again or you will have the same problem again.

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