AE explains valve breakage

Valve problems are often indicative of other engine issues. If a valve head or stem breaks, it is important to identify why.

The extreme conditions that modern engines have to work under place enormous stress on many engine components. Thus, if a component is faulty,  they can actually be a symptom of another fault. It is important to be aware of the core issue at hand.

AE demonstrates the different ways in which valves can be broken and the potential causes:

Valve head breakage

Valve head has broken away at the stem.

Potential causes:

  • Valve bounce caused by weak valve springs or engine over-speed
  • A seating velocity which is too high, due to excessive tappet clearance
  • Lost valve cap (if fitted)
  • Exhaust braking system malfunction – leading to valve float and piston/valve head collision
  • Excessive misalignment of valve guide and port seat – particularly if coupled with high engine speed
  • Piston/valve head collusion due to seized or sticking valve


Valve stem breakage (single-groove collet & multi-groove collet)

The square corners of the rectangular design can cause high stresses leading to breakage.
The free-rotating collet and stem arrangement is particularly susceptible to failure.

Other causes include:

  • Poor surface finish or damage in collet groove
  • Badly-seated collets
  • Valve bounce due to engine over-speed or weak valve springs
  • Damage during fitting

With so many different causes for valve failure, garages need to be ever vigilant during troubleshooting.

Available from FPS

More than 1,600 OE specification valve train parts covering over 35,000 applications are available from FPS via same/next-day delivery. For more information, please speak with your local FPS representative.


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