Radiator caps are an often-overlooked component of the cooling system that should be checked and replaced often. Higher cooling system pressure raises the boiling point of coolant to help reduce the risk of overheating. Caps allow coolant to exit the cooling system into an overflow or recovery bottle as coolant heats and expands. Recovery style caps work when the overflow hose is properly connected to the recovery bottle, as the system cools, coolant from the recovery bottle will be drawn back into the radiator to keep the cooling system full. Standard diameter caps fit 2-1/8" O.D. x 3/4" deep necks, and small diameter caps fit 1-5/8" O.D. x 5/8" deep necks (dimensions are approximate).