Chapter 5 Fundamentals of Engine Construction and Operation 77
from the cylinder head, between the valve stem and guide,
and into the combustion chamber. The valve seal may be
an umbrella type or an O-ring installed between the valve
stem and valve retainer.
When the valve is in contact with its seat, heat is
transferred from the valve to the cylinder head. This trans-
fer is most critical for exhaust valves, since they absorb
more heat of combustion. If the heat is not transferred
to the head, the valve may melt. Some valves are filled
with metallic sodium to further aid in heat transfer. The
intake valves are cooled by the incoming air and are not
as prone to damage.
The valve-to-seat contact is controlled by the valve
clearance. Valve clearance is the amount of loose-
ness in the valve train between the camshaft and valve
stem. Valve clearance can sometimes be adjusted. Valve
clearance has more effect on valve life than on engine
performance.
Valve Train
The valve train is the group of components that control
the opening and closing of the valves. Valve train opera-
tion is similar in both overhead camshaft and cam-in-block
engines. The engine crankshaft turns camshaft via a chain,
belt, or gear set. The camshaft controls the distance the
valves open and the duration of time over which they are
open. There is one camshaft lobe for each valve.
In a cam-in-block engine, the camshaft lobes push on
valve lifters installed into bores machined into the block.
The motion is transferred through push rods and rocker
arms to the valves. The valve spring pressure is overcome
and the valves are forced open. The valve remains open
Figure 5-6. The cylinder head contains the valves and acts as a cap on the top of the engine. Along with the cylinders, it forms
combustion chambers for each cylinder. (BMW)
Spark plug
Combustion
chamber
Exhaust
valves
Coolant passages
Intake valves
Figure 5-7. The valve assembly consists of the valve, valve
spring, retainers, stem lock, and oil seals. (Ford)
Split Keeper
Valve stem seal
Valve
Sleeve
Spring
Retainer