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how to replace a piston

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my bike 06 rmz 250 has 80 hr total 40 hr on the 2nd piston. just had it in the shop (not dealer) had valves checked still in spec. never shimmed. timing chain checked still in spec. i wanted him to do a leak down to check the piston he said he could not due to the auto decompress-or. after he fired it he said there is some minor piston slap and smokes. the piston slap i don't really hear but it does smoke but only when cold and with choke on. im not sure if i need a new piston or not? but with 40 hrs on it maybe i should replace it over the winter. (still has plenty of power and i only trail ride mostly with young kids) he wants 500$ ball park to replace it i would like to do it my self but im not sure how complicated it is on a 4 stroke i have done 2t when i was younger. how complicated are they to replace any one know of any videos out there? i did a search but no luck. any special tools required other then torque wrench and a fly wheel puller to do the cam chain? also should any thing else be done while in there

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the best piece of advice for you right now is to get a manual. there you will find step by step information on how to do these kinds of jobs as well as the necessary tools. top ends are actually quite easy, you just have to be patient and double check everything when you button it back up.

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like funkinalive said, if you get a manual, it is pretty easy. The only special tool that is needed for a piston is a ball honer, to hone the cylinder. If you have never replaced the valves and seals, I would think about it. You will need a valve spring compressor, lapping compound and tool, and a magnet tool for that (dont lap with the new valves). You can get piston/rings/gaskets for about $200, add in valves, seals, and springs you are up around $500.

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every bike does that with choke on, you have more fuel going into your carb, its not warmed up all the way so a hair bit of the fuel isnt being burned all the way.

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