Need some HELP ASAP Yz450F Rebuild!

Need some HELP ASAP Yz450F Rebuild!


Hey guys this is my first time rebuilding a YZ450f the bike is an 2006 I installed the new crank and have been putting everything back together! Well anyways I got the bike all put back together and I went to kick it over and NOTHING the kick starter wouldnt even budge. Well I tore it all back down thinking a ring must not be aligned correct on the piston. Well I took the top end apart and here is what I've got. The entire bottom end rolls over so smooth and easily but when I go to install the piston into the cylinder it fits but there seems to be alot of drag on the piston threw the cylinder!?!? Is that common am I missing something or what guys? It is a new wiseco piston I put the old one back in and same thing is happening, if you put a wrench on the crank shaft you can turn the engine over when the top ends on but it is not free rolling at all it to give you an idea of what the drag feels like it feels like your fighting a compression stroke constanly and that is with just the cylinder on not the upper head. PLEASE PLEASE PLEASE some help would be great!

maybe you need oil on the piston sides/cylinder wall? i wish i could help..but i really don't know

Sounds to me like your cranks not lined turns free w/out the jug because there is no drag, w/the jug on the piston isnt going straight up and down, its angled, un-aligned crank...did you use a jig?

That s what it sounds like to me...

Good luck w/that...


I really wish it was the crank not sitting at the correct angle. But the piston itself alone when triing to put it in cylinder on the bench has a ton of drag going up and down in the cylinder! I think the cylinder may be distorted at this point! Aghhhh

I think you should check the timing chain and guides/tensioners... Make sure the front tensioner is in correctly! and the auto tensioner isn't overloading the tension on the chain.

The piston with new rings will not run freely up and down the cylinder. It is. however, possible to improperly assemble the oil rings and run into trouble. Since you're fairly new to this kind of work, I'd suggest taking the cylinder and piston over to a shop and have someone give it a look. It's a little hard to help you more than that from here.

i rebuilt the motor in my '03 twice (the first time i forgot to install a circlip on one side of the piston wrist pin. argh! boy did that create a disaster the first time i fell over on the track towards that side and the wrist pin made contact with the cylinder wall!

anyways, double check the rings on the piston and make sure you are installing them with the right sides facing up. there should not be a reason why you can't kick the bike over. are there streaks on the cylinder walls from removing the piston from the cylinder after you reassembled it?

make sure you wear gloves and rub plenty of motor oil with your fingers tips on the cylinder walls when you are reassembling the motor. same thing for the wrist pin, oil it up good.

when you slide the cylinder back on again, don't put the head on just yet. hold the timing chain up with one hand, and with the other, see if you can manually push down on the kickstarter (very slowly) or have someone help you with that (easier way).

also, how are you lining up your cams? if you're not doing the cam timing right, that could prevent you from kicking also (which is a really, really good thing in that case). both lobes cant be up at the same time.

take pics of the cylinder wall for us, and take pics of how you lined up your cams when you put the chain back on.

unfortunately, i learned how to rebuild a yz450f the hard way! :-)

Thanks guys, Im not new to rebuilding enines by anymeans just never touched a 450f motor. Well anyways I put my bore micrometer down the cylinder and found it to be .80 out of round! And measure the old piston circumfrence and that was also ovalized and why it would fit. Got a new cylinder last night and rode it around at about 11 lastnight all is well! THANKS so much guys

Pistons are "cam ground", that is, they aren't cylindrical when they're new. They are (those that have a full skirt) oval shaped as viewed from the bottom, and barrel shaped as viewed from the sides. They are the largest at a point about level with the wrist pin centerline, and at 90 degrees to the pin bore (i.e., the front and the back).

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