Round, Steel, Zinc Oxide, Standard Size Radiator Necks, Each Higher cooling system pressure raises the boiling point of coolant to reduce the risk of overheating. Standard size caps fit 2-1/8" O.D. x 3/4" deep necks unless otherwise noted. Recovery system caps allow coolant to exit the cooling system into an overflow or recovery bottle as coolant heats and expands. As the system cools, recovery system caps will draw coolant back in to keep the cooling system full. Non-recovery system caps allow coolant to exit the system, but do not draw coolant back into the system.