4 – VALVE TRAIN
65
Piston Engine Fundamentals
TC010-05-01S
VALVE ADJUSTMENT
When a valve closes, there must be a small amount of clearance between
the end of the valve stem and the rocker arm. This clearance is called by
various names such as valve lash, tappet clearance, or valve clearance.
Engines that have solid lifters require periodic valve adjustments to make
sure the clearance is correct. The proper valve clearance eliminates noise
and excess friction.
In some engines, the clearance is adjusted by using shims, which are small
metal disks that come in various thicknesses. Figure 46 shows a tappet
and adjustment shim. By changing the thickness of the shim, the technician
adjusts the valve clearance.
Valve Clearance
The purpose of valve clearance is to make sure that the valve can close
tightly against its seat during compression and combustion. With no
clearance, the valve can be held slightly off its seat, causing compression
pressure to leak past the valve. When the valve remains partially open,
the valve head does not transfer heat to the seat, and combustion gases
will gradually burn the valve away.
On the other hand, if there is too much clearance, the valve train parts can
hammer against each other, causing a ticking noise often called ticking
valves.
FIGURE 46. In
some engines, a
shim is used to
adjust the valve
clearance.
Adjustment
shim
Tappet