CRACKING INTO VALVE
HEAD FAILURE
FEDERAL-MOGUL speaks to VITAL NEWS about
the various problems associated with valve heads
as a result of modern automotive technology.
Today’s downsized, turbocharged engines place enormous stresses
on many engine components. One component which can suffer from
these stresses more than others is the valve. Some of the most common
failures and their causes are listed below.
VALVE HEAD CHORDAL FAILURE
A
A classic valve head chordal failure
is one where the head breaks along
the B-X line as indicated in fig. 1.
The failure can also cause extensive
secondary damage to the piston,
cylinder head and even the cylinder
bore.
Chordal failure is usually caused by
fatigue as a result of stresses on the
valve from gas loading and high valve
head temperatures that the valve
head material is not strong enough
to cope with.
head will break off as a result. Other times, the crack will split to
opposite directions, causing two triangular shaped pieces to break off.
These pieces will typically disappear by the time a mechanic examines
the engine causing it to be misdiagnosed as a chordal failure. However,
the causes are vastly different.
M
X
B
Fig.1
M: Central Vertex/ Origin B-X: Chord
A-B: Diameter
B-M/M-X: Radius
More common in engines under
development than in service, valve head
chordal failures could be caused by;
• Excessive load on the engine
• Poor finish on the under-head radius
• Damage marks on the under-head radius
VALVE HEAD TULIPING
The head is cupped downwards in a tulip
shape, creating a poor seal that leads to power loss. This can be due to
conditions in the engine, engine over-speed, the design or materials of
the valve head – or any combination of the causes.
Once tuliping starts to occur, the valve
clearance would disappear. Valve tuliping
would affect the engine’s performance in
respect of poor idle, hesitation and possible
misfire. If not spotted in time, tuliping can
lead to chordal failure.
RADIAL RIM CRACKING
Typical causes include:
• Extreme temperature fluctuations as a result of excessive temperature and
pressure in the combustion chamber
• Continual engine overloading followed by abrupt unloading
• Poor temperature distribution across the valve and abnormal deflection of
the valve head due to excessive combustion pressures
• The rim or peripheral land of the valve head being too thin, with sharp edges
– usually due to too frequent reclaiming or regrinding
BACK-OF-FACE BURN THROUGH
Sometimes an exhaust valve will burn a
hole through the valve head, at the back
of the seating face. The failure would
have started with a rim crack, then
escaping combustion gases would have
rapidly eroded the valve head material.
SYMPTOMS AND CURES
With so many different ways for valve
heads to fail, it is important for garages
to identify the correct one to avoid future
occurrences.
More than 1,600 OE specification valve train parts covering over 35,000
applications are catalogued on MAM Autocat and available from FPS via
same/next-day delivery. For more information, please speak with your
local FPS representative.
Often known as thermal fatigue, radial rim cracking can occur on inlet
or exhaust valve heads. The crack starts in the rim of the valve and
travels inwards to the centre.
Sometimes the crack turns 90° across the valve head and back to the
valve seating face. As a result, a triangular-shaped piece of the valve
8 VALVE HEAD FAILURE
VITAL NEWS ISSUE 47