Valve clearance issue.

I recently checked the valve clearances on my 08 YZ450. Two of the valves were out of spec, middle intake and left exhaust. I got the correct shims and replaced them. I also replaced the piston, rings and timing chain at the same time. I put it back together and checked the clearances again. The two that I replaced the shims on are in spec but now the right exhaust is tight, .18 mm, it was .203 mm before. I tightened the cam cap bolts to 76 inch/lbs in three steps in the sequence that yamaha describes. I did clean the carbon off of the head with a scotch pad but didn't remove the valves to clean them. Why is this valve tight? Thanks.

Normally the last thing to do in a rebuild not the first.

Use some sand paper on a flat surface and take some off the shim. record what the shim was and what you sand it to for future reference.

Providing you put the same shim and bucket back in the same hole there is a very good chance you have some foreign matter {dirt} in there somewhere.

here are some pics of the cams and the timing mark, I believe everything is lined up.



Your timing is perfect.

The difference you're speaking of in the case of the exhaust valve is .02mm, which may seem like a lot when you read it, having been raised around things measured in thousandths of inches, but it's only .0008", and not even quite that. There could be any number of reasons for the discrepancy, including mixing lifters from left to right, the valve not being completely seated during the original measurement, temperature, or simple error in reading the feelers.

I don't recommend sanding shims, though. It's difficult to keep the two sides truly parallel by hand, and they're quite hard.

grayracer, would you recommend changing the shim? I had another person tell me that sometimes the clearances change because everything has been taken apart and that it will go back to where it was after the motor has been run and the lifters have been pressed more. I kept the shims and the buckets/lifters together and in order. It has a 180 in it now and I have a 175 to replace it with but I don't want to do that if it will loosen up too much. Thanks Grey.

The valves will sometimes settle in to tighter clearances, but rarely will they loosen. Still, it won't hurt to run it for an hour or so before checking things again.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now