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wr valves

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They all go through them whether you ride 'em fast or slow. WR and YZ is the same engine with different cams. If you keep them in adjustment they'll last longer.

Some have said that as the clearances close up, the cam no longer "sets" the valve on the seat, but allows it to fall harder onto the seat, accelerating their failure rate. Also as valves stretch they eventually exceed their elastic limit and "CRUNCH". That must be a sickening sound. Keep your eye on them, and if your shim sizes shrink by 10% to stay in spec, it's time for new valves.

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Some have said that as the clearances close up, the cam no longer "sets" the valve on the seat, but allows it to fall harder onto the seat, accelerating their failure rate.

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This doesn't make any sense to me.

When the clearances close up the valve no longer contacts the seat (this is why the first sign of tight valves is a loss of compression), the top of the valve stem will ride on the cam, allowing the valve face to 'burn' as it is exposed to the combustion process.

If the engine is over revved and valve float occurs then the valves can slam onto the seat, instead of riding the cam lobe.

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This doesn't make any sense to me.

When the clearances close up the valve no longer contacts the seat (this is why the first sign of tight valves is a loss of compression), the top of the valve stem will ride on the cam, allowing the valve face to 'burn' as it is exposed to the combustion process.

I think they were referring to the period of time between the valve clearance going out of spec and when it gets to zero clearance. The cam profile is such that it sets the valve down as long as there is sufficient valve clearance. After the valve is "supposed" to be seated, the cam profile is no longer tailored to gently seat it (if you can do anything gently 13,500 times per minute).

At least that's the way I understood it. If that's true, I can see how hammering the valves closed during this stage can accelerate wear/stretching. Obviously, as you say, valve float would be hard on valves too. I'm sure Rich Rohrich would be able to set us straight if we could get him to post up.

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