The radiator shroud ensures that the fan pulls air
through the radiator. It fastens to the rear of the radiator
and surrounds the area around the fan. When the fan is
spinning, the shroud keeps air from circulating between
the back of the radiator and the front of the fan. As a
result, a large volume of air flows through the radiator
The water passages in the cylinder block and
cylinder head form the engine waterjacket (fig. 64). In
the cylinder block, the water jacket completely
surrounds all cylinders along their full length. Within
the jacket, narrow passages are provided between the
cylinders for coolant circulation around them. In
addition, water passages are provided around the valve
seats and other hot parts of the cylinder block In the
cylinder head, the water jacket covers the combustion
chambers at the top of the cylinders and contains
passages around the valve seats when the valves are
located in the head.
The passages of the water jacket are designed to
control circulation of coolant and provide proper
cooling throughout the engine.
The pump forces
coolant directly from the lower radiator tank connection
into the forward portion of the cylinder block. This type
of circulation would, obviously, cool the number one
cylinder first; causing the rear cylinder to accept coolant
progressively heated by the cylinders ahead. To prevent
this condition, the L-head block is equipped with a
coolant distribution tube that extends from front to rear
of the block, having holes adjacent to (and directed at)
the hottest parts of each cylinder. I-head engines are
equipped with ferrule type coolant directors that direct a
jet of coolant toward the exhaust valve seats.
Automatic control of the temperature of the engine
is necessary for efficient engine performance and
If the engine is allowed to
operate at a low temperature, sludge buildup and
excessive fuel consumption will occur. On the other
hand, overheating the engine or operating it above
normal temperature will result in burnt valves and
The latter causes early engine
The thermostat senses engine temperature and
controls coolant flow through the radiator. It allows
coolant to circulate freely only withinthe blockuntil the
desired temperature is reached. This action shortens the
warm-up period. The thermostat normally fits under the
thermostat housing between the engine and the end of
the upper radiator hose. The pellet-type thermostat that
is used in modern pressurized cooling systems
incorporates the piston and spring principle (fig. 6-11).
The thermostat consists of a valve that is operated by a
piston or a steel pin that fits into a small case, containing
a copper impregnated wax pellet. A spring holds the
piston and valve in a normally closed position. When
the thermostat is heated, the pellet expands and pushes
the valve open.
As the pellet and thermostat cools,
spring tension overcomes pellet expansion and the
Thermostats are designed to open at specific
temperatures. This is known as thermostat rating.
Normal ratings are between 180°F and 195°F for
automotive applications and between 170°F and 203°F
for heavy-duty applications. Thermostats will begin to
open at their rated temperature and are fully open about
20°F higher, For example, a thermostat with a rating of
195°F starts to open at that temperature and is fully open
at about 215°F.
Most engines have a small coolant bypass passage
that permits some coolant to circulate within the
cylinder block and head when the engine is cold and the
thermostat is closed. This provides equal warming of
the cylinders and prevents hot spots. When the engine
warms up, the bypass must close or become restricted.
Otherwise, the coolant would continue to circulate
within the engine and too little would return to the
radiator for cooling.
The bypass passage may be an internal passage or
an external bypass hose. The bypass hose connects the
cylinder block or head to the water pump. There are two
internal bypass systems that can be used on an engine.
One internal bypass system uses a small, spring-
loaded valve located in the back of the water pump. The
valve is forced open by coolant pressure from the pump
when the thermostat is closed. As the thermostat opens,
the coolant pressure drops within the engine and the
bypass valve closes.