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Bent Exhaust Valve

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#1
Greg Kimble

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I recently had an engine loose power at the end of a race.  After the loss of power, the engine still ran, had good oil pressure, but sounded like a lawn mower.  I began to tear down the motor yesterday and found that one of the exhaust valves on the #4 cylinder had come in contact with the piston and was bent.  I do not see any other damage in the cylinder.

 

This motor as many (too many) races on it.  What has to fail in order for the valve to hit the piston?  Timing belt looks fine and as I mentioned above engine would run after the incident.  Also should I expect to find any other damage in the motor due to the piston and valve colliding?

 

Any insight is appreciated

 

Thanks

 

Greg


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#2
shedguy

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Hey Greg, Most likly the spring gave up. 


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#3
chris haldeman

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Inspect valve guide after disassembly. Often when the valve bends and tries to close it breaks the bottom of the guide
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#4
chris haldeman

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As shed guy mentioned plus a few causes...
Human error shifting
Valve spring failed
Valve keepers failed
Valve spring retainer failed
Tip of valve broke off at keeper.
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#5
Ron Alan

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My failure was the last on Chris's list! Valve falls..gets hit and bent by piston...the bend gets shoved back into the guide and stuck! This all happens in the flash of a spark plug!

 

And other than the guide and a mark on the piston...no real other damage. 


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#6
38bfast

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over rev?


Ralph Provitz
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#7
Steve Scheifler

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over rev?


I’d guess that’s it roughly 95% of the time on these engines, typically an early downshift or wrong gear entirely, nicknamed a money shift for good reason.
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#8
Steve Scheifler

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I should add that the over rev doesn’t necessarily cause immediate failure. A slightly bent valve can hang causing the lash pad to be hammered by the cam lobe rather than follow it, and cause the valve to seat unevenly producing additional stresses and overheating due to hot exhaust getting by it, until one end or the other fails.
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