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When our bodies feel cold, we put on a jacket. Our car engines wear
permanent jackets for the opposite reason-- to keep cool!
The water jacket is a collection of passages within the block and
head. These passages let the coolant circulate around the "hot spots"
(valve seats and guides, cylinder walls, combustion chamber, etc.) in
order to cool them off.
The engine block is actually manufactured in one piece with the water
jackets cast into the block and cylinder head. At normal operating
temperature, the water pump forces the coolant through the head
gasket openings and on into the water jackets in the cylinder head.
It flows around in there, cooling everything off by absorbing the
heat. After doing its thing, the coolant flows through the upper hose
to the radiator where it releases the heat. Then, the water pump
sends it back down into the engine's water jackets to continue the
cooling process.
On the sides of the engine are "freeze" or "expansion" plugs, which
are sheet metal plugs pressed into a series of holes in the block.
These are designed to hold the pressure of the cooling system, but to
pop out if the coolant in the block ever freezes.
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