04 yz450f motor problems

Im new to Thumpertalk and i recently bought a 2004 YZ450F. It ran great for about two weeks then i was out riding it one day and it acted like it ran out of gas. I checked gas and it was fine. i got it back to the house and tryed to start it. It hardly had any compression and you could take your hand and kick it over. Tore it apart and the piston was cracked nearly in half. Took it to the machine shop and they said all the valves were messed up. I got a new piston, valves, and gaskets and took the valves to be put in and check the clearence on the new piston. The clearence varies a lot going down the jug so it needs bored. What could be the possible cause of this?? There isnt any play in the crank up or down. Has anyone ever had this problem before?

The bore is not perfectly the same top to bottom. The top part of the bore will be tighter then the bottom of the bore. Picture a construction cone. Now if the bore is shaped like a oval, or something then yeah it is messed up. In this case you either buy a new cylinder, or bore perfectly round then re plate to the correct piston to cylinder clearance.

No its perfectly round as far as i can see. How much difference should there be from the top to the bottom and do you have any idea as to why the piston cracked?

Thanks for the reply it was much appreciated

You have to use a bore indicator, micrometer or another tool that can measure the bore to .001". You can't "see" it with your eye.

As far as cracked pistons, if it was a stock piston I have seen extended use or "time" as a factor in pistons cracking. Other then that detonation can cause a piston to crack. Can you post pictures of the piston and these messed up valves?

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