when to replace a piston

i have a08 yz450 and was wondering what the sign of you needing to replace the piston are?

Any help would be appreciated as i am clueless about pistons

Tear down, inspection, and measurement are the only sure ways to know, unless there are some outward symptoms of trouble.

you can run the comprassion test just to get an idea at first.

what should the compression be when cold?

Compression tests are fairly useless in an engine with automatic decompression. They're fantastically inaccurate in engines without it. Leak down tests are far more reliable. Have one done at a good shop.

Tear down, inspection, and measurement are the only sure ways to know, unless there are some outward symptoms of trouble.

So, what would the outward symptoms of trouble be?

So, what would the outward symptoms of trouble be?

Burning oil and smoking, poor performance, and possibly some noise.

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.

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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