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?