GMC 1990 Light Truck Repair Manual page 474

6A4-2 4.8 LITER L6
SUBJECT
Assembly of Engine (Cont.)
Oil Pan Installation....................................................................................................................................................6A4-28
Cylinder Head Installation....................................................................................................................................... 6A4-29
Valve Train Component Installation.......................................................................................................................6A4-29
Valve Adjustment......................................................................................................................................................6A4-29
Checking Valve Tim ing........................................................................................................................................... 6A4-30
Rocker Arm Cover and Tappet Cover Installation..............................................................................................6A4-30
Water Pump Installation......................................................................................................................................... 6A4-30
Thermostat Housing Installation...........................................................................................................................6A4-30
Intake and Exhaust Manifold Installation.............................................................................................................. 6A4-31
Air Manifold Installation......................................................................................................................................... 6A4-31
Flywheel Installation............................................................................................................................................... 6A4-31
Engine Accessory Installation...............................................................................................................................6A4-31
Engine Set-Up and Testing..................................................................................................................................... 6A4-31
Specifications..............................................................................................................................................................6A4-32
Special Tools................................................................................................................................................................ 6A4-35
Figure 1—Lubrication Diagram (Side View)
GM 4.8L engines are inline six cylinder type, overhead
valve, water cooled, with cast iron block and head.
The crankshaft is supported by seven precision insert
main bearings, with crankshaft thrust taken at the number
seven (rear) bearing.
The camshaft is supported by four plain type bearings
and is gear driven. Motion from the camshaft is transmit­
ted to the valves by hydraulic lifters, pushrods, and ball-
type rocker arms. The valve guides are integral in the
cylinder head.
CONTENTS (CONTINUED)
DESCRIPTION
The connecting rods are forged steel, with precision in­
sert type crankpin bearings. The piston pins are a press fit
in the connecting rods.
The pistons are case aluminum alloy. The piston pins
are a floating fit in the pistons.
Lubrication schematics are shown in figures 1 and 2.
The gear type oil pump is driven through an extension
shaft from the distributor drive shaft, which is gear driven
from the camshaft. Oil is drawn into the oil pump through
a pickup screen and pipe (figure
routed to the oil filter. In case of excessive backpressure
at the oil filter, a bypass valve is provided. This valve will
allow oil flow to the main oil gallery at the right side of the
block (figure 1). This gallery supplies oil to the camshaft
bearings, hydraulic lifters, and main bearings. The con­
necting rods are supplied oil from the main bearings by
means of drilled passages in the crankshaft. The valve
train is supplied with oil by the hydraulic lifers. Oil is
pumped from the lifters through the hollow pushrods to
the rocker arms. Oil drains back to the crankcase through
oil drain holes and the pushrod holes. The piston, piston
pin, and timing gears are lubricated by oil splash.
For engine identification, refer to GENERAL INFORMA­
TION (SEC.
2
). Pressurized oil is
6
A).
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