SUPERTECH PERFORMANCE
Breakage - thermal overstress
Failure caused by wide variations of temperature and pressure within the combustion chamber.
Result
Valve head breaks, along a chord of its circle, by the under head radius (the fracture starts with a fissure in the radius).
cause
Very high pressures and temperatures in the combustion chamber. (These problems are mainly associated with the exhaust valves).
contributing factors
- Engine overspeed. Valve float.
- Weak valve springs.
- High seating speed.
- Abnormal combustion.
- 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
- Thermal shock.
- Engine overload.
- Excessive combustion temperature and pressure.
- Weak valve springs.
- Seating velocity too high.
Mechanical overstress
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
- Weak springs. Valve float.
- Engine over speed.
- Seating velocity too high due to excessive lash.
- Incorrect valve train clearance.
- 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
- Defective keeper assembly.
- Keeper groove damages.
- Valve float.
- Excessive valve train clearance.
- Engine over speed, especially on valves with rectangular keeper grooves.
Breakage
Impact failure
Characteristic appearance of an impact failure. NOTE: Breakage by impact is a sudden failure. (example: Piston / Valve colli-sion).
valve burning
Result
Valve face burnt and torched
cause
Excessive localized heat in the valve head, distortion and seat leakage (poor seating).
contributing factors
- Lack of stem to guide clearance.
- Worn valve guide and/or misalignment of valve stem and guide.
- Pre-ignition (lean air fuel mixture, incorrect fuel).
- Improper compression ratio.
- Defective cooling system. f Incorrect lash adjustment.
- Excessive carbon build up on valve.
Fatigue failure
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.
valve face pitting
Result
Pitted valve and valve seal faces.
cause
Solid particles pressed between valve face and valve seat.
contributing factors
- excessive oil consumption (through piston rings, valve guides and valve stem seals).
- Abnormal combustion.
- Long idle periods.
- Thermostat malfunction (below normal engine temperature).