A radiator hose is a one that transfers coolant from an engine's water pump to its radiator. It is connected to a nipple on the radiator or the engine's water pump or intake by a clamp. Most radiator hose designs are molded hoses specific to the application; however, there are some universal designs which can be bent and fit onto many different applications.
When replacing a radiator hose, it is wise to also replace the hose clamp. There are many types of radiator hose clamps in use, from the single strand of heavy spring steel that is installed with a pair of pliers, to the more common screw type that is installed and tightened with a screwdriver. Some return radiator hose designs utilize a steel spring inside of the hose to prevent it from being sucked together by the force of the water pump.
In most applications, the return radiator hose is the lower or bottom hose on the radiator. Coolant is sucked out of the radiator by the water pump through this hose and it enters the engine block. The coolant is pushed through the engine block and into the heads by the water pump. Once the coolant has finished its journey through the engine, it has absorbed a great deal of heat. It is then pushed out of the engine and sent back into the radiator to begin its cooling cycle.
There are as many different dimensions of hose as there are designs. While each vehicle has its own specific design of hose with bends in just the right places, so to does each vehicle utilize its own size of hose. There are hoses as small as 1-inch (2.5 cm) diameter all the way to hoses with diameters of several inches. The rate of flow that is mandated by the time it takes to draw the heat out of the engine dictates the size of hose needed to obtain that rate of flow.
An engine that has its coolant circulating through it and radiator too quickly will overheat. The coolant needs to have time in the engine to absorb the heat as well as time in the radiator to expel it. The size of the radiator hose can help to achieve this time, which is known as saturation time. Often the bottom radiator hose is a larger diameter than the upper hose. This allows the water pump to pull a large amount of coolant out of the radiator, but the smaller hose on the top prevents the coolant from escaping the engine too quickly.