Breakage - Thermal Overstress
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.
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.
a.) Thermal shock.
b.) Engine Overload
c.) Excessive combustion temperature and pressure
d.) Weak valve springs
e.) Seating velocity too high
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.
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.
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
Characteristic appearance of an impact failure.
Note: Breakage by impact is a sudden failure
Example: Piston / Valve collision.
Characteristic appearance of a fatigue failure
Note: Breakage by material fatigue happens after thousands of cycles, starting in a small defect and advancing until breakage occurs.
Result: Valve face burnt and torched
Cause: Excessive localized heat in the valve head, distortion and seat leakage (poor seating).
a) Lack of stem to guide clearance.
b) Worn valve guide and/or misalignment of valve stem and guide.
c) Pre-ignition (lean air fuel mixture, incorrect fuel).
d) Improper compression ratio.
e) Defective cooling system.
f) Incorrect lash adjustment.
g) Excessive carbon build up on valve.
VALVE face pitting
Result: Pitted valve and valve seal faces.
Cause: Solid particles pressed between valve face and valve seat.
a) excessive oil consumption (through piston rings, valve guides and valve stem seals).
b) Abnormal combustion.
c) Long idle periods.
d) Thermostat malfunction (below normal engine temperature).