upper end of the cylinders and, in air-cooled engines,
the cylinder heads are bolted to the top of the cylinders.
This serves to provide a combustion chamber (fig. 3-11)
for the ignition of the mixture and to hold the expansion
forces of the burning gases so they may act on the
piston. In a gasoline engine, there are threaded holes to
position the spark plugs in the combustion chamber. On
a diesel engine, there is a similar arrangement to
position the fuel injectors. In a liquid-cooled engine, it
also contains passages, matching those of the cylinder
block, that allow cooling liquid to circulate in the head.
The I-head (overhead valve) type of cylinder head
(fig. 3-12) contains not only water jackets for cooling
spark plugs openings, valve pockets, and part of the
combustion chamber, but it also contains and supports
the valves and valve operating mechanisms. In this type
of cylinder head, the water jackets must be large enough
to cool not only the top of the combustion chamber but
also the valve seats, valves, and valve operating
The cylinder heads are sealed (fig. 3-13) to the
cylinder block to prevent gases from escaping. This is
accomplished on liquid-cooled engines by the use of a
head gasket. The head gasket is usually made of two
sheets of soft steel that sandwich a layer of asbestos.
Steel rings are used to line the cylinder openings. They
are designed to hold the tremendous pressure created on
the power stroke. Holes are cut in the gasket to match
the coolant and lubrication feed holes between the
Figure 3-11.Combustion chambers.
cylinder head and the cylinder block. In an air-cooled
engine, cylinder heads are sealed to the tops of the
cylinders by soft metal rings. The lubrication system
feeds oil to the heads through the pushrods.
Figure 3-12.Cylinder head for overhead valve engine.