VALVE FACING
PROBLEM
Valve face burning and guttering are serious problems which can lead to
cracks in or breakages from the cylinder head. FEDERAL-MOGUL speaks
to VITAL NEWS about the types and causes of valve face problems.
Valve face wear can be serious, but mainly occurs in older engines since
modern fuel does not provide the lubrication they require. Valve face
burning or guttering – where a deep channel is formed in the valve
head- can happen to any engine, so it’s worth being aware of the causes.
WHERE’S THE WEAR?
Older engines rely on exhaust gases to lubricate the valve faces.
However, modern low-leaded and LPG fuels don’t provide enough
lubrication resulting in wear, especially in an engine with slack tappets,
weak springs, or valve to port seat misalignment.
In turbocharged petrol engines,
wet fuel is a sufficient lubricant;
however turbocharged diesel engines
can suffer from face wear. Since
turbocharging raises air pressure
in the inlet port above normal
atmospheric air pressure, lubricating
oil isn’t taken into the air stream.
This causes the valve face and seat to
run dry enough for wear to occur.
A BROADER ISSUE
Guttering on the inlet valve is known as broad-face burning. Material
corrodes over the valve face, resulting in an escape of gas affecting
performance.
This is most likely to affect non-hard-faced valves and can be
caused by:
• Valve head/seat misalignment – resulting from inherent valve bridge
weakness, engine overload, or valve head distortion.
• Weak valve springs
• Movement of the valve seat insert
LACK OF TAPPET CLEARANCE
Lightly sitting valves allow carbon to build up on the seating face,
which destroys the thermal heat path to the coolant. With nowhere
for the heat to escape to, the temperature of the valve head and face
rise until hot corrosion or burning takes place, creating the gutter.
HIGH CARBON DEPOSIT BUILD UP
Over-rich combustion and excessive oil consumption can also create
carbon deposits. This again destroys the thermal path to the coolant,
resulting in valve burn out. If deposits break away it can cause a
localised gas leakage which accelerates the failure.
TORCHING DURING EROSION
Nickel-alloy is used for the valve faces for its high hot-corrosion
resistance. Yet the alloy is weak in the erosion phase, which can lead
to torching. This is a rarer occurrence, but can be identified by the
deep gutter caused by the local gas leak.
PRE-IGNITION
An incorrect air/fuel ratio, inaccurate timing, or incandescent hot
spots on the valve head periphery can all produce high gas loads,
leading to localised high temperatures. Thi s results in guttering,
although the piston can sometimes fail before guttering occurs.
PREVENTION IS BETTER THAN CURE
Manufacturers attempt to prevent this kind of exhaust valve face wear
with amended valve geometry and more wear-resistant materials for
the cast iron head.
For effective prevention in all cases, it’s important to fit and adjust
carefully and accurately. Installers are advised to use parts from
VITAL NEWS ISSUE 52
reputable manufacturers, which incorporate the latest proven
materials, design innovations and manufacturing techniques.
AE valves are available for same/next-day from FPS via the F:Drive,
and MAM Autocat. For more information, please speak to your local
FPS representative.
VALVE WEAR 13