have to use something to prevent loss of liquids as the
and coolant tight seal. These gaskets may be metal
engine operates. Seals or O-rings are used to seal the
sheets soft enough to deform as required, but they are
clearances between the moving and stationary parts and
usually made in the form of two thin plates of soft metal
the outside of the engine.
with a heat-resistant filling between them. Holes are
punched in the gasket to conform to the openings in the
Seals manufactured today are too numerous in
head and block, which allows the coolant to circulate. If
design to attempt coverage of each particular design in
a poorly made or improper gasket is used, the flow of
this text. We will mention the more common types and
coolant may be restricted, causing troublesome hot
their use only. Most seals you will become familiar with
spots.
are made of neoprene rubber, molded or crimped to a
metal backing. These are known as lip-type seals. The
On some engines, especially V-type, the gaskets are
metal seals the outside diameter of the hole it is used in
so nearly uniform that they can be turned upside down
by a press fit. The neoprene, which is flexible and gives
or end for end and still fit the retaining studs and
a snug fit around the moving part, seals the inside to
combustion chambers. If they are improperly installed,
prevent leakage.
however, they will stop coolant circulation at some part
of the engine. It is important to make sure the correct
O-rings, another type of seal, are used primarily in
gasket is used and that the side marked UP or TOP is
places where the clearance is considerably smaller than
facing away from the block. If no markings are found,
with lip-type seals. As the parts are installed, the O-ring
the first surface of the gasket is placed against the
is squeezed to form a tight seal.
block.
On most gas and diesel engines, the rear main seal
The intake and exhaust manifold gaskets are
is a two-piece strip of molded neoprene rubber, which
usually made from asbestos formed to the desired
is squeezed around the rear of the crankshaft between
shape. Some are metal covered and similar in
the engine block and rear main bearing cap. This seal
construction to a head gasket. Because of the heat of the
works in the same manner as the O-ring.
exhaust and intake air, it is necessary that manifold
As you gain experience in the mechanical field, you
gaskets be made from a material that will not burn.
will be able to recognize the different types of seals and
The oil pan gasket is generally made from pressed
how they work to prevent leaks. In the meantime, if you
cork. It may be made in one piece but more often it is
discover a seal that you have a question about, ask your
made in two pieces--one piece for each side of the pan.
supervisor. Your supervisor will be able to explain how
Each end of the pan is sealed with cork or neoprene
seals work and why they are made differently.
gasket material.
MOVING PARTS OF AN ENGINE
The other gaskets for an engine are usually cut from
pressed paper that is oil resistant. The thickness of the
The moving parts of an engine serve an important
paper varies in accordance with manufacturer's
function in turning heat energy into mechanical energy.
recommendations. Often the gaskets used will create
They further convert reciprocal motion into rotary
specified clearances between stationary parts of an
motion. The principal moving parts are the piston
engine.
assembly, connecting rods, crankshaft assembly
When you install any gasket in an engine, it is
(including flywheel and vibration dampener),
important for the surfaces to be sealed and to be clean
camshaft, valves, and gear train.
and free from grit and parts of the old gasket. A putty
The burning of the fuel-air mixture within the
knife is useful for cleaning gasket surfaces. You must
cylinder exerts a pressure on the piston, thus pushing it
be sure the gasket is in the proper position and does not
down in the cylinder. The action of the connecting rod
slip when the two parts are brought together. Heavy
and crankshaft converts this downward motion to a
grease or a gasket compound will help you make sure
rotary motion.
the gaskets stay in place and form a good seal.
Piston Assembly
Seals
Engine pistons serve a number of purposes. They
As explained above, gaskets prevent leaks between
transmit the force of combustion to the crankshaft
stationary parts. As these gaskets are not made to
through the connecting rod; they act as a guide for the
withstand movement of the engine components, you
upper end of the connecting rod; and they serve as a
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