Water (Coolant) Jackets

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.