Four-stroke combustion engines are amazing works of engineer- ing. The level of precision, quality
of parts, and special treatments have
significantly improved during the past
20 years. We are able to put our vehicles
through more abuse: higher rpm, greater
heat, increased loads and leaner fuel
mixtures, while achieving amazing performance. In this way, I’m glad EPA has
pushed the OEMs to be cleaner on emissions. It has forced the manufacturers to
tighten tolerances, use better materials
and improve efficiencies to maintain
compliance. As good as they are, they are
not bulletproof and will eventually wear.
Over time, our valve trains may need
adjusting or replacement.
All valves open and close once every
two rotations of the crank. During one
crank rotation, the intake valves open and
close (PHOTO 1). During the next crank
rotation, the exhaust valves open and
close. At idle speed, our engines generally
turn about 1,200 rpm. That means during
one minute of idling, the valves open and
close 600 times. At 12,000 rpm an engine
opens and closes the valves 6,000 times
per minute or 100 times per second. With
such a narrow window, timing is crucial.
Four-stroke engines use a camshaft
(cam) to transfer rotational movement
from the crank, to up-and-down movement for the valves. Intake valves control
air/fuel charge moving into the cylinder.
Exhaust valves open to expel spent
gasses from the cylinder. If the cam
timing is just a little off, performance
will be affected. It could cause a stumble
off idle, more fuel consumption or loss
of power. If the timing is way off, it can
cause a piston to collide with the valves,
causing catastrophic damage.
Valve clearance inspections check the
gap between valve-actuating parts. To
perform the clearance check, the crank
needs to be rotated to Top Dead Center of
Compression ( TDCC) (PHOTO 2) on each
cylinder checked. This is usually done
through an access hole with timing marks
aligned between the rotor and case. With
the engine cold, the technician will use
feeler gauges to check the gap (PHOTO 3).
Clearance is required to allow for heat ex-
pansion. As the engine warms up, the gap
is reduced. The gap also affects timing of
the valves. If the gap is too large, the valves
will open later. If the gap is too small, the
valves will open sooner. The designed
gap is just a few thousandths of an inch
or hundredths of a millimeter. To put this
into perspective, it’s about the thickness
of standard copier paper, or a couple of
human hairs in width.
As components wear, some will cause
the clearance to increase and others
will cause a decrease. In most cases,
the largest contributor to clearance
will be the valves and seats. After the
valves are opened, a spring or rocker
arm forces the valve closed very quickly.
When this occurs, the face of the valve is
hammered into the seat of the cylinder
Valve performance is vital to maintaining a
smooth-running engine and saving you money.
Here’s what you need to know about the process.
> text and photos
by Kevin O’Shaughnessy
36 MCN I For Enthusiasts MCNEWS.COM
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