The adjustment of valve clearances can generally
pushrod contacts the valve stem. The valve adjusting
be changed by an adjustment screw in the rocker arm or
device, in this case, is in the rocker arm.
by a threaded part of the pushrod (fig. 3-41). A locknut
Many engines have self-adjusting valve lifters of
is provided with either device. The manufacturer's
the hydraulic type that operate at zero clearance at all
manual gives you the proper setting and the valve
times. The operation of one type of hydraulic valve
tappet mechanism is shown in figure 3-40. Oil under
pressure is forced into the tappet when the valve is
HYDRAULICALLY OPERATED VALVE AD-
closed, and this pressure extends the plunger in the
JUSTMENT.--On engines equipped with hydraulic
tappet so that all valve clearance, or lash, is eliminated.
valve lifters (fig. 3-42), it is not generally necessary to
When the cam lobe moves around under the tappet and
adjust the valves. They operate with zero clearance (fig.
starts to raise it, there will not be any tappet noise. As
3-40). The engine lubrication system supplies a flow of
the lobe starts to raise the tappet, the oil is forced
oil to the lifters at all times. Hydraulic lifters
upward in the lower chamber of the tappet. This action
compensate for changes in engine temperature, adapt
closes the ball check valve so oil cannot escape. Then,
automatically for minor wear at various points, and thus
the tappet acts as though it were a simple, one-piece
provide ideal valve timing.
tappet and the valve is opened. When the lobe moves
If for any reason the valve mechanism must be
out from under the tappet and the valve closes, the
repaired, an initial adjustment must be made after
pressure in the lower chamber of the tappet is relieved.
reassembling the job. Almost all manufacturers have a
Any slight loss of oil from the lower chamber is then
different procedure for making this adjustment, so
replaced by the oil pressure from the engine lubricating
check the manufacturer's service guide for the engine
system. This causes the plunger to move up snugly
against the pushrod so that any clearance is eliminated.
HYDRAULIC VALVE LIFTER SERVICE.--
ADJUSTING VALVE OPERATING MECH-
Hydraulic valve lifters are comparatively simple in
ANISMS.--The intake and exhaust valves are
design. Servicing the lifters requires only that care and
operated by rocker arms and pushrod assemblies
cleanliness be exercised when handling parts.
working off the engine camshaft. In some engines, the
The first indication of a faulty lifter is that it starts
camshaft acts directly on the rocker arm to open the
making a "clicking" noise. A listening rod or an engine
valves. In all cases, it is important that the proper
stethoscope is helpful in locating a noisy valve lifter.
clearance (valve lash) be maintained in the linkage to
You do this by placing one end of the tool against the
compensate for expansion as the engine heats up. Too
particular part of the engine and the other part to your
much clearance causes noise. Also, the valves do not
ear. Move the instrument around the engine until you
remain open long enough. Too little clearance prevents
have narrowed the noise down to a certain area. In an
proper seating and causes burned and leaky valves.
Figure 3-40.--Operation of a hydraulic valve tappet or lifter.