Bent Valves - How?

so recently my 2003 YZ 450 bent the 3 intake valves.

However, the cam chain was replaced in January 08, along with the piston and rings. upon inspection, the tensioner is working fine, not as strong as the new one i got for it but other than that it works proper.

I work at A yamaha Dealer, and some people have said the cam seized in the journals, thus causing the chain to jump and the piston to hit them, and my machinist ( Wally Routledge) says the piston hit the valves, causing the cap to bend a tiny bit, seizing the cam after the initial hit ( hes the one who repaired it)

I did replace the clutch friction plates, steel plates, and springs about a month ago ( about 20 hours use before valve bending) , anyhow i found out earlier this week that i accidentrly installed a WR426 clutch....

my hinson clutch basket shows a bit of wear, but not as much as i see in the bottom of the cases and in the oil filter

so the question ihave , is why did the timing mess up, causing the bent valves, and why is theree so much sediment?

if you need any more info please ask, but thanks for any help!

A while back, I had a bit of gunk grab hold of the valve stem which held the valve open long enough to hit the piston. It happened when the engine was at high revs. They were intake valves also.

you think that clutch plates caused your valves to get bent???


A very common cause of cam seizure is incorrect torque on the cam caps. Torquing them out of sequence, too much, or all in one step can and will warp the cap and cause this. There are other things that can lead to it as well, but clearance issues and such would be more apt to show up early in the bike's life than later.

The piston cannot have hit the valves first, as there is nothing other than cam timing that would have caused it, other than a broken valve spring, and it that case, the impact would not have been felt by the cam.

well ill tell ya what happened

i had about 20 hours on the new clutch , about 90 hours on the rebuild i mentioned that i did last january....

anyhow 5 minutes into a ride it made a loud skreech, and locked up.... bike wasnt fully warmed up yet, and the revs were very low.........hmmmm

you think that clutch plates caused your valves to get bent???


sacrastic or serious???

mod`s :blah:

The same thing just happened to my son's 2003 250F. He was going very slow-we were at the start of an enduro,when it just died. Upon inspection, I found all 3 intake valves bent.The cam chain appeared ok,as did the tensioner,but the cam lobes were both facing backwards!Obviously had skipped several teeth.The intake cam,when compared to the new one,looks like it is one tooth off. I have just spent a small fortune rebuilding the engine, but I still don't know for sure that it won't do it again.

mod`s :thumbsup:


Anyway, it sounds all the more as if the cam seized. And 314 brings up a point I forget too often, which is that when the cam seizes, it tends to rotate the cam sprocket on the camshaft itself. Even if the cam looks usable, it should be very carefully compared with a new one to verify that this has not happened.

Once it's running again, do a check of top end oil delivery by using the oil pressure check bolt on the head.

ok ok, i lined up the cams on my head, and the intake lobes are pointing directly to the right (looking at the cam sproket side) and the exhausts are pointing left but also up a bit, like kind of in a left-up, direction.....

is this seeming normal? also should I consider getting a new intake cam???

is this seeming normal? also should I consider getting a new intake cam???

Don't consider it, do it. The cam slipped in its sprocket.

My orig. cam sounds about the same. I put the old cam in a brass-jawed vise, and tried to turn the sprocket with a large pair of channel-locks, just to see if it would turn. But I could not budge it,but it got out of time somehow.Do many of you spot weld the gear?

spot welding the gear has been common practice since '98.

Thanks guys so much!!! I have been chasing why my 2006 YZ450 had no power after the chain slipped and this explained it. After looking at my intake cam at TDC, it is clear the sprocket has moved.

Thanks again everyone for the help!!!



does anybody know if spot welding the gear on the cam will cause an in-balance in the cam and cause vibration??

If the welder puts two reasonably similar spots on opposite edges of the shaft, it won't be a problem. Cams turn at half the speed of the engine, and the welds will be very near the center, so any asymmetry would have little effect.

It should be noted that the sprockets rarely slip unless the cams seize first.

Whenever it's ALL intake or ALL exhaust valves that are damaged, you can be 100% sure your cam seized as Gray said. Every time it's all valves (I or E), it ties it back to the cam.

If you get one valve bent it can be due to spring failure, grime holding valve open or individual valve failure.

Pistons don't just hit valves and THEN mess up your timing - that's Physically impossible unless your piston broke off the conrod.

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