Q: how to reinstall cams/camchain on '03 yz450?


i'm doing a top-end rebuild (1st time for me on a 4stroke motor) on my '03 yz450f.

i've got the piston at TDC and the camchain tensioner is out of the engine. my question is, the camchain barely fits over both cams.. how do you guys line up/install the cams when rebuilding? the yamaha manual says to do the exh cam first, but it's pretty difficult either way..

is there some trick you guys use? or is it just 'fiddle with it till ya get it in' (no wisecracks please, i see the door wide open on that one )?

thanks in advance.

A cam chain that's close to the right (non-worn out) length can't be lifted off either cam once the cams are in place. Even with only one in position, there is not really adequate room to shift the chain along the cam sprocket. What needs to be done is to unbolt the cams and tip the one you want to reposition out of place enough to move the chain one or whatever number of teeth it needs to correct its timing. It's a little tricky, but that's the only way.

The exhaust should be done first, but it doesn't have to be. It does have to be in its journals with the chain over it though, because the only way to correctly time the intake is to have the chain running over the exhaust sprocket, and all the slack out of the front and top runs of chain, so it usually works out easier to do the exhaust first. When you're done, double check by backing the engine up carefully, then rotate it forward to TDC while holding pressure on the tensioner guide with a finger through the back of the cylinder. Check the marks, and if it's good, install the tensioner.

thanks for the reply. just got finished putting it back together...

the method i used was:

1) put the exhaust cam on in place

2) with the exhaust cam in, put the intake cam on as close to the exhaust cam as possible (chain slacks the most closest to exh cam)

3) since the cam rolls on a bearing, just roll the intake cam into place

the only catch was you have to guess how many teeth will roll by as you roll the cam into place.. i had to re-position the cam 3-4 times before i got it right, but all-in-all it was pretty painless.

reassembled the bike with the new top-end and she fired right up on the first kick.

this was my first time rebuilding a 4 stroke top-end. it was easier than i thought it would be. :applause::D:prof::prof:

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