Failure caused by mechanical origin. This includes wear and breakage, which has nothing to do with the combustion chamber environment.

Result: Valve head to stem breakage.

 

Cause: Repeated stem stress.

 

Contributing Factors:

a.)    Weak springs. Valve Float

b.)    Engine over speed

c.)     Seating velocity too high due to excessive lash

d.)    Incorrect valve train clearance

e.)    Engine over speed, especially on valves with rectangular keeper grooves.

Result: Valve stem breakage in the keeper groove area.

 

Cause: Material fatigue due to mechanical hardening on the surface. It occurs on the contact zone between grooves and keepers.

 

Contributing Factors:

a.)    Defective Keeper assembly

b.)    Keeper groove damages

c.)     Valve float

d.)    Excessive valve train clearance

e.)    Engine over speed, especially on valves with rectangular keeper grooves