A radiator header tank is an integral part of a modern automobile's cooling system. It has an important role in dealing with the heat generated from a vehicle's internal combustion engine through the successful continual circulation of coolant.
The purpose of a radiator header tank is to provide a place for coolant to expand and condense before being recirculated, and thus to prevent the loss of coolant, which can be expensive. For a long time, until the 1950s, overheated coolant was simply allowed to evaporate, meaning that the radiator had to be refilled regularly.
The radiator header tank has been installed in virtually all cars starting in the 1960s. The tank will be located at the top of the radiator system, above the expansion tank, and it almost always is visible from any point above.
Because the purpose of a radiator reserve tank is to deal with high temperature, high pressure coolant, it can sustain a lot of wear and tear through years of use and extended driving, especially in hot weather. On older cars, the radiator reserve tank may have been added later. Many of these are made entirely of plastic, which does not age or wear well, and should be checked regularly. If an engine shows a tendency to overheat, the first -- and easiest -- place to check is the radiator reserve tank.