The cylinder head (fig. 3-9) provides
combustion for the engine cylinders. It is built to conform to the arrangement of the
valves: L-head, I-head, or others. Cylinder heads on liquid-cooled engines have been made
almost exclusively from cast iron until recent years. Because weight has become an
important consideration, a large percentage cylinder heads now are being made from
aluminum. The cylinder heads on air-cooled engines are made exclusively from aluminum.
This is due to the fact that aluminum conducts heat approximately three times as fast as
cast iron. This is a critical consideration with air cooling.
liquid-cooled engines. the cylinder (fig. 3-10) head is bolted to the top of the
cylinder block to close the 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.
(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 mechanisms.
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.
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 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.
3-10.Cylinder head for L-head engine.
3-12.Cylinder head for overhead valve engine.
3-13.Cylinder head sealing.