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ENGINE DIVISION SERVICE MANUAL
TM 5-4210-230-14&P-1
ENGINE
Hydraulic Valve Lifters (Tappets)
The hydraulic valve lifter or tappet consists of a body
(cylinder), plunger, plunger spring and check valve assembly
and is removed from the engine as an assembly. The lifters
are removed after removing the valve cover, rocker arms and
push rods.
NOTE:
If a tappet cannot be removed due to
carbon buildup, use a carburetor solvent
in the tappet bore. However, if any
excessive amount of solvent is used, it will
be necessary to change the oil.
Optional sources of hydraulic valve lifters have been
established. When disassembling the lifters, do not mix the
components of any lifter. Figure 120, illustrates the difference
in construction between types "A" and "B." Generally, the
service instructions are the same for all types of hydraulic
lifters. The push rods are common for all types of lifters.
Fig. 120 Sectional View of Hydraulic Tappets Showing Types
"A" and "B"
1.Type A
6.
Plunger oil hole
2.Type B
7.
Plunger 3.
Snap ring
8.
Check valve 4.
Identification rules
9.
Spring 5.
Tappet oil hole
10. Oil Chamber
Operation of Valve Lifter
Oil from the engine lubricating system is fed to a
supply chamber in the hydraulic lifter plunger from which it
can flow directly through the plunger, past the check valve
and into the compression space between the bottom of the
body and the plunger. When the engine is started, oil
pressure is built up in the system which fills the supply
chamber. With the lifter on the base circle of the cam and the
engine valve seated, the light plunger spring moves the
plunger outwardly to remove all lash from the valve gear. As
the plunger moves outward increasing the volume in the
compression chamber, the check valve moves off its seat and
the chamber is supplied with oil. As the camshaft rotates and
lifts the body upward, a pressure is immediately built up in the
compression chamber, forcing the valve onto its seat. Further
rotation of the camshaft lifting the valve gear against
increasing valve spring pressure increases the oil pressure in
the compression chamber, thus acting as a solid member of
the valve operating mechanism. As long as the engine valve
is off its seat, the load of the mechanism is carried by the
column of oil. During initial running of the engine, there might
be some air remaining in the compression chamber which,
under pressure, tends to leak through the clearance between
the plunger and cylinder, thus decreasing compression space
volume. However, when the lifter is again on the base circle of
the cam, the plunger moves outward, drawing in more oil so
that after a number of cycles, the air is completely eliminated
and a solid column of oil is maintained in the compression
space. During this period when some air is present in the
compression space, the valve gear may be somewhat noisy,
since it has the same effect as excessive lash with aerated oil.
However, once the unit is completely cleared of air, it then
becomes a true hydraulic system.
The compensation required to take care of variations in length
from the cam to the valve seat is accomplished by the original
fit of the plunger in the cylinder bore. These parts are
selectively fitted to give a predetermined slight leakage of oil
(called leakdown) under load. The purpose of this
predetermined leakage is to permit the valve to seat
irrespective of the change in length between the cam and
valve seat. If the valve gear is expanding, thus reducing the
distance between the cam and valve seat, the load imposed
by the valve spring on the lifter causes the above mentioned
slight leakage between the plunger and cylinder, reducing the
clearance volume and, therefore, the overall lifter length.
When
CGES-210 Page 44
PRINTED IN UNITED STATES OF AMERICA
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