2004 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? :excuseme:

Has anyone ever had this problem before?
Nope, you're the first. :excuseme:

The cylinder can't be bored, exactly, because it's a plated aluminum bore. You have two practical choices:

  1. Replace the cylinder with a new one.
  2. Refinish your old one.
  3. Buy a big bore kit.

In the second process, the old plating is honed out, new plating applied, and that layer is then diamond honed to the original size.

like i said i bought the bike used and come to find out it has a 97mm(+2mm) piston over stock so it has been messed with before. Are all the yz450f cylinders tighter at the top than at the bottom?

Not when they're new, no. They're supposed to be straight, top to bottom. Max dimensional variation from a perfect cylinder is .002"

.002 all way way down the cylinder? even at the bottom? i heard it was always tighter at the top than at the bottom but it was on the internet that i heard that. Would there being too much clearance between the piston and clyinder be what cracked the piston?

.002 all way way down the cylinder? even at the bottom? i heard it was always tighter at the top than at the bottom but it was on the internet that i heard that. Would there being too much clearance between the piston and clyinder be what cracked the piston?

Automotive engives are slightly tappered but not your YZ

No cylinder is ever tapered as manufactured. Normal wear will produce a taper, but the taper will end wear the top piston ring reaches TDC, so the very top of the cylinder should show almost no wear, even on a high hour engine. If you call straight forward 12:00 o'clock, then most cylinders will show little to no wear at the bottom of the bore when measured across 1:30/7:30 or 10:30/4:30 either, because nothing ever touches it there.

Either way, all new cylinders are round horizontally and straight vertically, and nay more than .002" variation from that is over the limit on a YZ450.

well what I meant was not the part of the cylinder the piston contacts but the fact that auto engine pistons can not be removed from the bottom due to design. I thought maybe thats what the o/p read.

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