Question:I drive an older import car. My owner's manual
says I'm supposed to have the valves adjusted every 25,000 miles.
What will happen if I don't?
Answer: Nothing, for awhile. But eventually the valve
lash on your engine will loosen up or close up. On valves where
it loosens up, it will make noise and accelerate rocker arm and
valve tip wear. On valves where the lash closes up, it will cause
the valves to leak and burn.
Periodic valve lash adjustments are required because most
of these engines have mechanical lifters or cam followers. These
were used to cut costs (and boost performance, some might argue).
Hydraulic lifters are more complicated and expensive, but run
much quieter and eliminate the need for period valve lash adjustments.
Hydraulic lifters or followers are now used on almost all engines.
But if you're driving an engine that doesn't have them, you'll
have to live with periodic valve lash adjustments or suffer the
consequences.
VALVE ADJUSTMENT
The valves are adjusted on pushrod engines by changing the
height of the rocker arm or turning an adjustment screw on the
end of the rocker arm. The clearance between the tip of the rocker
arm and valve stem is adjusted to a specified clearance which
is measured with a feeler gauge. The spec may require the lash
to be set with the engine hot or cold. It makes a difference
so always refer to a shop manual for the exact clearances and
adjustment procedure.
It's also important to note that the specified valve lash
for exhaust valves is often greater than that for intake valves.
This is necessary because exhaust valves run hotter than intake
valves and grow longer due to thermal expansion when the engine
heats up.
Each valve must be adjusted with the piston at top dead center
so both valves are closed and the lifters are resting on the lowest
portion of the camshaft. The best approach is to start with cylinder
number one, then proceed in the engine's firing order.
