Pontiac FIREBIRD 1972 Service Manual page 562

6-50 1972 PONTIAC SERVICE MANUAL
Fig. 6-8 1 Rear Main Bearing Cap
inertia member. The accessory pulleys are bolted to the
harmonic balancer and may be changed independently of
each other.
The rear main bearing is sealed by a packing (all but 307
engine), or neoprene seal (307 engine) seated in a chamf-
ered groove in the block and bearing cap (Fig. 6-81). A
slinger on the crankshaft in front of the seal and the drain
groove in the rear main bearing prevent an excess of oil
from getting to the seal.
Two cavities are drilled in the cylinder block and cap seal
groove to prevent seal rotation.
CAMSHAFT AND DRIVE
The engine chart shows the application of camshafts
which can be identified by a letter stamped on the front
end of the shaft.
Camshafts are cast from alloy iron and cam lobes are
ground, hardened and tapered with the high side toward
the rear. This, coupled with a spherical face on the lifter,
causes valve lifters to rotate. The camshaft is supported by
five bearings.
A 7/8" wide, 60-link timing chain is used to drive the
camshaft. The 42-tooth camshaft drive sprocket is made
from hardened cast iron while the 2 1-tooth crankshaft
sprocket is made from hardened sintered iron.
PISTONS AND CONNECTING RODS
Pistons are of a light weight cast all aluminum slipper skirt
type. Their light weight results in reduced inertia forces.
These pistons maintain thermal control through contour
grinding. The taper cam grind provides quiet piston oper-
ation and adds to overall engine smoothness. Two com-
pression rings and one oil control ring are used, all of
which are located above the piston pin. Machined reliefs
on the top of the piston contribute to the determination of
compression ratio and provide for valve clearance (Fig.
6-82).
Fig. 6 - 8 2 Piston Identification
Piston pins are offset toward thrust side (right-hand side)
to provide a gradual change in thrust pressure against the
cylinder wall as the piston travels its path (Fig. 6-83). This
feature provides quieter engine operation. Pins are hard-
ened steel and have a floating fit in the pistons. They are
retained in the connecting rods by a press fit.
A lubrication groove between the connecting rod and cap
directs a jet of oil onto the opposite cylinder wall to help
lubricate the piston and rings during initial cold drive
away operation.
VALVE TRAIN
A very simple ball pivot-type valve train is used (Fig.
6-84). Motion is transmitted from the camshaft through
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CENTERLINE
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RIOHT 8 M K LEFT BANK
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Fig. 6 8 3 Connecting Rod and Piston Assembly