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when to replace a piston


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alot of the old bike repair manuels I've read though list, poor starting as a possible sign of a bad piston or rings. Don't over think your way into a repair that didn't need to be made. I've seen alot of yamaha's with a bunch of hard hours on them, that haven't been touched.

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You can buy a leak down tester for fairly inexpensive, but the percentage of leakage may not mean a whole lot to you unless you have a new engine to compare it against. Once you see a significant decrease in your leak percentage then you can determine if it's time to rebuild. A leak down tester will also help you diagnose if the compression loss is due to a piston/rings that aren't sealing as well as they used to, or if you have a leak somewhere else, like your valves or a bad head gasket.

I did one on my Husky SMR 510 that I used to have, and was leaking 12%, which I was told was ok to keep running for another season, but without a comparison percentage from when the motor was fresh, who knows.....

that said pistons are cheap, if you have 200 hours or so on your bike, just spend the $150 and do it

here's one on ebay http://cgi.ebay.com/ebaymotors/2006-2009-YAMAHA-YZ450F-TOP-END-PISTON-KIT-W-GASKETS-OE-/110441537034?pt=Motorcycles_Parts_Accessories

not sure why they show a 2 stroke piston in the pic :banana:

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