Failure caused by wide variations of temperature and pressure within the combustion chamber.

Results: Valve head breaks, along a chord of its circles, by the under head radius (the fracture starts with a fissure in the radius).

 

Cause: Very high pressure and temperatures in the combustion chamber. These problems are mainly associated with the exhaust valves.

 

Contributing Factors:

a.)    Engine Overspeed. Valve Float

b.)    Weak valve springs.

c.)     High seating speed.

d.)    Abnormal combustion

e.)    Incorrect Fuel.

Result: A radial crack inward from the margin. If the fissure advances, the head will break.

 

Cause: Thermal fatigue due to high temperatures and unequal temperatures in different zones of the valve head.

 

Contributing Factors:

a.)    Thermal shock.

b.)    Engine Overload

c.)     Excessive combustion temperature and pressure

d.)    Weak valve springs

e.)    Seating velocity too high