THE STEM OF
VALVE PROBLEMS
FEDERAL-MOGUL shares
some troubleshooting
pointers on valve stem
and guide failures.
Excessive valve stem and guide wear can occur on inlet and exhaust
valves. Whilst there is a large number of potential causes for failure,
some are actually symptoms of another problem. Whilst some causes
are easier to spot than others, the most likely causes for excessive valve
stem and guide wear include:
Improper fitting Poor lubrication
Too much stem-to-guide
clearance can lead to the
bell-mouthing of the guide.
Misalignment of valve-guide-to-
seat results in high-side loading. Lack of oil or a breakdown of
the oil film can result in stem
scoring and scuffing. Temporary
poor lubrication coverage
when cold starting in sub-zero
temperatures can be similarly
problematic.
Incorrect valve height after an
overhaul can lead to excessive
side thrust on rocker-operated
valves. The valve will be heavily
recessed because too much
metal has been removed from
valve seat and face. Incorrect
fitting of special high-lift cams
may also be a cause, as they
affect the rocker geometry.
Valve timing problems
Bent valve stems due to slipped
or incorrect valve timing, or
piston-to-valve collision due to
over-speed in valve bounce.
Contamination
Trapped foreign particles cause
abrasive wear. This can occur
particularly after an overhaul,
when residual caborundum
particles are present. Too much
carbon packing at the guide’s
port-end results in stem scuffing.
STICKING OR SIZED VALVE STEM
The above issues can lead to other problems.
For example, a bent valve stem will lead to
stem seizure. However, this can also be caused
by lack of lubrication, too little stem-to-guide
clearance, or a damaged stem.
is a symptom of another issue which can lead to bigger problems if the
root cause is not identified.
Root causes include:
• Worn guides
• Excessive stem-to-guide clearance
• Wrong lubricating oil grade
• Overloaded engine
• Too much cold-running under a light load
• Oil seal failure – leading to formation of excessive oxidised lubricating oil
on the valve stem
• Weak or broken valve springs
VALVE TIP WEAR
A badly-worn valve tip can be a symptom of various potential problems.
Installers are advised to lookout for:
• Whether the valve tip is matched with
a worn rocker pad
• Rocker misalignment
• Incorrectly hardened tip-end
• Faulty lubrication or wrong grade of
lubricating oil
• Valve springs are too strong
• Incorrect timing that forces the
exhaust valve to open against
excessive back pressures
• Excessive tappet clearance
Troubleshooting valve train issues requires the
installer to be aware that a valve train problem
is likely a symptom of the root cause. Thus, it is
imperative to resolve the core issue in order to
ensure long-lasting results upon repair.
More than 1,600 OE specification valve train
parts by AE valves are available from FPS via
same/next-day delivery. For more information,
please speak with your local FPS representative.
A valve stem tends to stick in extreme
temperature conditions – due to gumming
deposits when it is cold, or carbon packing
between the stem and guide when the engine
is hot. Either way, the sticking of a valve stem
16 ENGINE VALVES
VITAL NEWS ISSUE 49