A water pump moves the coolant through the passages (water jackets) in the engine cylinder block and cylinder head and then out into the radiator. This helps to remove the excessive heat from the engine; the hot coolant passes through the radiator where it cools down and then returns back to the engine. A water pump consists of the housing with the shaft rotating on the bearing pressed inside. At the outer side there is a pulley mounted on the shaft. At the inner side there is a seal to keep the coolant from leaking out and the impeller that acts like a centrifugal pump.