Chevrolet 1977 light duty truck, Service Manual

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outer ring o f the d a m p e r is grooved for the accessory
drive belts.
CAMSHAFT AND DRIVE
The cast iron c a m sh a ft is supported by four
bearings and is g ear driven. A cast iron c ra n k sh a ft gear
drives the alu m in u m c a m sh a ft gear. Cam lobes are
ground, h a rd e n e d and tapered w ith the hig h side toward
the rear. This, coupled with a spherical face on the
lifters, causes the valve lifters to rotate.
PISTONS AND CONNECTING RODS
The pistons are m ad e o f a cast alu m in u m alloy
using two compression rings an d one oil control ring.
Piston pins are offset .060" toward the thrust side
(right h a n d side) to provide a gradual change in thrust
pressure against the cylinder wall as the piston travels its
path. Pins are c h ro m iu m steel an d have a floating fit in
the pistons. T hey are retained in the connecting rods by
a press fit.
C onnecting rods are m ade o f forged steel. Full
pressure lubrication is directed to the connecting rods by
drilled oil passages from the adjacent m ain bearing
jo u rn a l. Oil holes at the connecting rod journals are
located so that oil is supplied to give m axium u
lubrication just p rio r to full bearing load.
VALVE TRAIN
A very simple ball pivot-type train is used. Motion
is transm itted fro m the c a m sh a ft through the hydraulic
lifters and push rods to the rocker arms. The rocker arm
pivots on its ball and transm its the ca m sh a ft motion to
the valve. T he rocker a rm ball is retain ed by a self
locking nut.
HYDRAULIC VALVE LIFTERS
H ydraulic Valve Lifters are used to keep all parts o f
the valve train in constant contact for quiet operation.
T he hydraulic lifter assembly consists of: the lifter
body, which rides in the cylinder block boss, a plunger, a
push ro d seat, a m eterin g valve, a plunger spring, a
check ball and spring, a check ball retain er and a push
rod seat retainer.
W h e n the lifter is rid in g on the low point (base
circle) o f the cam, the p lu n g er spring keeps the plu nger
an d push rod seat in contact with the push rod.
W h e n the lifter body begins to ride up the cam lobe,
the check ball cust off the tran sfer o f oil from the
reservoir below the plunger. T he plunger and lifter body
th en rise as a unit, pushing up the push rod and opening
the valve. A very small am o u n t o f oil will leak out
between the plunger and the body.
As the lifter body rides down the o th e r side o f the
cam, the plunger follows with it until the valve closes.
T he lifter body continues to follow the cam to its low
point, but the plunger spring keeps the plunger in
contact with the push rod. The ball check valve will then
move off its seat and the lifter reservoir will be refilled
with oil.
INTAKE MANIFOLD
The intake m anifold is o f cast iron, single level
design for efficient fuel distribution. The 250 m anifold is
an integral unit with the head, except KIO and CIO with
F44, while the 292 uses a separate com ponent. The
carb u reto r pad is centrally located with a passage
run n in g u n d e rn e a th the pad (E.F.E.), through which
exhaust gases are forced, to prom ote faster fuel
vaporization w hen the engine is cold. An E G R port is
also cast into the manifold for the induction o f a
m etered am ount o f exhaust gases into the air and fuel
mixture which has entered through the carburetor.
EXHAUST MANIFOLD
A single four port, underslung, center take down
m anifold o f cast iron is used to direct exhaust gases from
the combustion chambers. A heat shield is m ounted to
the m anifold that is used to route heated air to the air
cleaner for better fuel vaporization.
ENGINE SERVICE
NOTE: The following in fo rm atio n is im portant in
preventing engine d am ag e
and in providing
reliable engine perform ance.
W hen raising or supporting the engine for any
reason, do not use a jack u nder the oil pan, an y sheet
metal or crankshaft pulley. Due to the small clearance
between the oil p a n and the oil p u m p screen, jacking
against the oil p a n m ay cause it to be bent against the
pu m p screen, resulting in a dam ag ed oil pickup unit.
It should be kept in mind, while working on the
engine, that the 12-volt electrical system is capable o f
violent and d a m a g in g short circuits. W h e n p e rfo rm in g
any work where electrical term inals could possibly be
grounded, the ground cable o f the battery should be
disconnected at the battery.
Any time the c arburetor o r air cleaner is removed,
the intake opening should be covered. This will protect
against accidental entrance o f foreign m aterial which
could follow the intake passage into the cylinder and
cause extensive d am ag e w hen the engine is started.
Engine Lubrication
Full pressure lubrication, through a full flow oil
filter is furnished by a gear-type oil pum p. The
distributor, driven by a helical gear on the camshaft,
drives the oil pum p. The m a in oil gallery feeds oil,
through drilled passages, to the c am sh aft and cran k sh aft
to lubricate the bearings. The m ain oil gallery also feeds
the valve lifters; which, through hollow push rods, feed
the individually m ounted rocker arms (Fig. 6A-3).