Ideas on what caused this?

2 replies to this topic
  • desertwheelspin

Posted October 06, 2015 - 06:22 PM


Coming off of a jump, bike stalled and kick starter wouldn't move.  Tore down the top end and found this.  I am curious to hear any theories on what may have caused this.  It's an 08 yz450f.  





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  • SDet

Posted October 06, 2015 - 06:24 PM


Fatigue stress caused valve to break

Impact between valve an piston sent it into the head

Luck lodged it in the valve hole. 

  • grayracer513

Posted October 07, 2015 - 08:32 AM


In most cases, dropping a valve head like that is caused by one of two things:


Wear resulting in fatigue.  When the valves are allowed to wear badly at their face (they have been reshimmed beyond 0.10 mm smaller than their original shim size), they can begin to "scrub" lateral to the stem axis as they seat because the wear does not always develop concentrically between face and seat.  That can work the stem to the point of breaking off at the valve head.


Timing.  When the cam chain becomes stiff at the link joints, there are a number of situations, such as brake stomping the engine over a jump, in which the engine speed is suddenly changed that can cause the defective chain to skip time, which can then lead to a valve collision with the piston, often breaking one or the other of both parts.  Cam seizures can do this, also. 

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