Result:
Valve face burnt and torched. Cause:
Excessive localized heat in the valve head,
distortion and seat leakage (poor seating). Contributing Factors:
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
Result:
Wide areas of valve face burnt but not torched
through. Cause:
Excessive accumulated heat on the valve head,
concentrated on the margin.
Contributing Factors:
a)
Worn valve guide.
b)
Excess material removed during a previous re-facing
operation.
c) Poor
valve seating.
Result:
A hole burnt through the back of the valve
head into the underhead radius just behind the
valve face. This is more common on hard faced
valves. Cause:
Excessive localized heat in the valve head,
distortion and seat leakage (poor seating)
It starts as a radial rim crack or thermal
fatigue, then the base material burns through
behind the hardened face.
Contributing Factors:
a)
Engine overload. b) Fuel system
problems (lean air-fuel mixture).
c)
Pre-ignition.
d)
Incorrect fuel.
e)
Poor valve seating.
Valve face Pitting
Result:
Pitted valve and valve seat faces Cause:
Solid particles pressed between valve face and valve
seat Contributing Factors:
a)
Excessive oil consumption (through piston rings,
valves guides and valve stem seals)
b)
Abnormal combustion.
c)
Long idle periods. d)
Thermostat
malfunction (bellow normal engine temperature).
Corrosion
Result:
1) Corrosion on the valve underhead between
the neck and the valve face.
2) Valve stem necking. Cause:
Erosion and corrosion caused by exhaust gases.
Contributing Factors:
a)
Use of inadequate valve material.
b)
Excessive engine overload conditions (Overheating).
c)
Incorrect fuel.
d)
Lean air-fuel mixture.
Distortion
Result:
Cupping or tuliping of the valve head. Cause:
Very high seating forces. Excessive
combustion temperature and pressure.
Contributing Factors:
a)
Engine overload.
b)
Pre-ignition.
c)
Detonation.
d)
Excessive valve spring pressure. e) Improper
compression ratio.
f)
Improper air-fuel mixture.
g)
Difference in angle between the valve seat and the
valve face.
Seizure or Scuffing
Result:
Stem seizure or scuffing Cause: High
temperature caused by friction due to lack of
clearance and/or lubrication Contributing Factors: a) Poor stem to guide
clearance
b)
Insufficient lubrication.
c)
Valve underhead, neck, and stem carbon build up.
d)
Engine running at a low speed or overloaded.
e)
Valve stem bent (Possible collision with the piston).
f)
Incorrect valve stem seals.
g)
Falta de alineación entre vástago,
guía y asiento de la tapa.
*
Wear
Result:
Valve stem and guide wear
Cause: Excessive stem to guide clearance. Signs of
high temperature and stem/guide seizure.
Contributing Factors:
a) Poor to stem to guide clearance (oil film
breakdown)
b) Excessive stem to guide clearance (insufficient
heat dissipation)
c) Poor stem lubrication
d) Wrong valve stem seals
e) Incorrect rocker arm geometry
f) Restricted exhaust flow
*
Result:
a) Guide out-of-round due to wear
b) Valve stem uneven wear.
c) Valve tip wear
Cause: High temperature caused by friction and
misalignment of the valve stem.
Contributing Factors:
a) Worn rocker arm pivot
b) Improper valve tip grinding
c) Guide/seat misalignment
d) Excessive lifter bore clearance
e) Too much spring tension
f) Insufficient lubrication
*
Result:
Valve facing grooving
Cause:
a) Improper fuel .
b) Incorrect Springs
Contributing Factors:
a) Weak valve springs.
b) Valve seat/guide misalignment .
c) LPG or propane gas used as fuel.
Result:
Valve wear in the keeper groove area (single or
multi-groove).
Cause: Use of worn or improper keepers
Contributing Factors:
a) Worn keepers
b) Too much spring tension or defective springs
c) Worn valve retainers
d) Insufficient lubrication
e) High speed seating generating excess friction
between the retainer, keepers, and valve.
Important:
These are the most common examples of valve
related failures
Since the least common result of valve failure is
the valve itself, it is always necessary to
determine the cause of failure prior to
replacement of the damaged valve(s).
The information herein submitted was compiled with
utmost care, based upon the most updated and reliable
sources and is herein given for information purposes
only. Basso S.A. assumes no responsibility for eventual
mistakes.
Undue use of the primary information herein given is the
sole responsibility of the user.
Any reader of this issue is hereby notified that he/she
may reproduce or use the information herein contained
only if expressly authorized by Basso, provided that the
information source is duly mentioned.
Basso S.A. - Martín Oliver y Ruta 70 S - (2300) Rafaela
- Santa Fe - ARGENTINA
Tel: (54 - 3492) 434421 / 435421 / 425979
Result:
Valve stem and guide wear 
Result:
a) Guide out-of-round due to wear 
Result:
Valve facing grooving
Result:
Valve wear in the keeper groove area (single or
multi-groove).
