76 Auto Engine Performance and Driveability
shaft by a bearing cap and bearing inserts that surround the
crankshaft journal. The piston pin and crankshaft bearings
allow the rod to move in relation to both the piston and
crankshaft. Refer to Figure 5-4.
The crankshaft converts the straight-line force from the
piston and connecting rod into rotary force. It is attached
to the engine block by bearing caps and bearings that sur-
round the crankshaft journal, Figure 5-5. This design allows
the crankshaft to rotate inside of the bearings with minimal
friction. The bearing caps are held to the engine block by
two, four, or six bolts torqued to specifications.
Cylinder Heads
The cylinder head contains the combustion chamber
for each cylinder and forms the top of the cylinder. Cylinder
heads contain the intake and exhaust valves and, in some
cases, the camshaft and lifters. They also contain oil gal-
leries, coolant passages, and openings to allow the flow of
intake and exhaust gases. Cylinder heads are made from
either cast iron or aluminum. A sheet metal, cast aluminum,
or plastic valve cover is installed over the upper valve train
components. Figure 5-6 shows a typical cylinder head.
Coolant passages between the cylinder heads and
engine block must be sealed to prevent coolant leakage.
Also, the pressure of expanding combustion gases must
be contained within the cylinder. Head gaskets are used
between the head and engine block for these purposes.
They are thin and made from steel, copper, and fibers.
The cylinder head and head gasket are secured to the
block with head bolts. These must be torqued to specifica-
tions. A few engines have studs and nuts, rather than head
bolts.
Valves and Related Components
One or more intake valves are used to control the flow
of the air into each cylinder. One or more exhaust valves
are used to control the flow of exhaust gases out of each
cylinder. Valves also seal the cylinder during the com-
pression and power strokes. They are occasionally called
mushroom valves due to their resemblance to a mushroom.
Intake and exhaust valves are identical in shape, but intake
valves are usually larger. Opening and closing of the valves
are controlled by the valve train.
The valve spring holds the valve against its seat, keep-
ing it closed. Valve springs are always slightly compressed
when installed. This ensures that the valve closes tightly.
The spring is held to the valve by valve spring retainers. The
retainer is a cap which covers the spring. A locking device,
usually called a split keeper, locks the cap to the valve stem.
A valve and spring assembly is shown in Figure 5-7. The
assembly is held together by the pressure of valve spring
acting against the cylinder head.
The valve stem slides up and down in a valve guide.
The guide may be integral to the cylinder head or a remov-
able insert. It keeps the valve steady and provides a smooth
surface on which the stem can slide. The oil seal at the top
of each valve stem prevents engine oil from entering the
combustion chamber. Without a seal, oil would be pulled
Figure 5-5. The crankshaft is the engine part that converts the
piston’s up-and-down (reciprocating) motion into rotary motion.
It delivers power to the drive train. (Ford)
Make sure the tab
on the bearing
fits tightly into
the matching
notch in the cap
Make sure the tab
on the bearing
fits tightly into
the matching
notch in the block
Thrust
bearings
Main
bearing
caps
Bearing
inserts
(lower)
Bearing
inserts
(upper)
Crankshaft
Figure 5-4. The connecting rods attach the pistons to the
crankshaft. (Ford)
Piston
Bolt
Nut
Connecting
rod
Bearing
inserts
Piston
Pin