Valve Train - Ford LSG 875 Service Manual

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BASIC ENGINE
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DESCRIPTION AND OPERATION (Continued)
VALVE TRAIN
The camshaft is supported by five bearings
pressed into the block. It is driven at one-half
crankshaft speed by the timing chain and
sprockets. The camshaft sprocket
is
positioned
by a dowel in the forward face of the front
camshaft bearing journal. Camshaft end play is
controlled by a thrust plate attached to the front
of the cylinder block. An eccentric
cam
for fuel
pump actuation is
attached
to the front face of
the sprocket. A helical accessory drive gear is
machined in
the
camshaft directly behind the
front journal to drive the distributor and oil pump.
(There are two camshafts available for the 460
marine engines depending upon whether the en-
gine is standard rotation or reverse rotation.)
Hydraulic valve lifters ride directly on the
camshaft lobes and transmit the thrust of the
lobes to the push rods which actuate the valve
train.
When a valve is closed, the related push rod
is in its lowest position and the lifter assembly is
on the base circle of the cam lobe. The valve
lifter plunger spring expands, forcing the plunger
upward. This force is transmitted to the rocker
arm through the push rod, causing solid contact
between the valve end of the rocker arm and the
. valve stem.
As the valve lifter plunger spring expands to
move the plunger upward, the volume of the
compression chamber in the bottom of the lifter
body is increased. Oil, supplied at full lubrication
system pressure through the oil gallery pas-
sages in the cylinder block unseats the disc type
check valve in the bottom of the lifter and enters
the compression chamber to fill the increased
volume. The check valve closes when the cham-
ber
is
filled.
As the camshaft rotates, the lifter body is rais-
ed by the cam lobe. The oil in the compression
chamber, being incompressible, acts as a solid
member and transfers the lifting force to the
plunger and push rod. Because of the load im-
posed on the plunger by the push rod, the oil in
the compression chamber is subjected to in-
creased pressure. This causes a slight leakage
out of the chamber past the walls of the plunger.
The leakage flow is known as the calibrated
leak-down rate and is controlled by precise
matching of valve body and plunger during origi-
nal assembly of the valve lifter. Consequently,
individual hydraulic valve lifter components are
not interchangeable.
When the high point of the cam rotates past
the foot of the valve lifter body, the lifter is forced
downward by the push rod as the valve (intake
or exhaust) returns to its seat in the cylinder
head. This reduces the force on the lifter plunger
and allows the plunger to be raised once again
by the plunger spring. Engine oil is forced into
the compression chamber to replace that which
leaked out, priming the lifter for its next operat-
ing cycle.
Hydraulic force and plunger spring action in
the valve lifter take up all clearances in the valve
train mechanism to maintain zero valve lash.
The push rods are tubular steel with ball ends.
Each push rod receives oil from a disc-type
metering valve in the push rod cup at the top of
the valve lifter. The oil enters and leaves the
push rod through holes in both ball ends to inde-
pendently lubricate each rocker arm.
The rocker arms
each
have a hole in their
push rod end for lubrication. Each rocker arm is
pedestal mounted. The rocker arm pivots on a
spherical fulcrum. A bolt retains the rocker arm
and fulcrum on the head.
Intake and exhaust valves are of forged alloy
steel with chrome plated stems. Both intake and
exhaust valves are the free turning type. Be-
cause the valves are canted, they open obliq-
uely into the combustion chambers, in the
direction of gas flow, to improve engine
breathing. Pockets are cast in the piston heads
to provide clearance at top dead center for full
valve opening.