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. ALL30124 Recovery System Radiator Cap Small Stant 16 PSI ALL30125 Recovery System Radiator Cap Small Stant 19-21 PSI ALL30126 Recovery System Radiator Cap Stant Round Small Dia. (32mm I.D., 16mm deep) ALL30128 Non-Recovery Radiator Cap Stant 28-32 PSI ALL30129 Recovery System Radiator Cap Round 29-31 PSI ALL30130 Recovery System Radiator Cap Stant w/Lever 18-22 PSI ALL30131 Recovery System Radiator Cap Stant w/Lever 21-25 PSI ALL30132 Recovery System Radiator Cap Stant Round 16 PSI ALL30133 Recovery System Radiator Cap Stant Round w/Lever 16 PSI ALL30134 Non-Recovery System Radiator Cap Stant 18-22 PSI ALL30135 Non-Recovery System Radiator Cap Stant 21-25 PSI ALL30136 Recovery System Radiator Cap 22-24 PSI ALL30137 Recovery System Radiator Cap Stant Octagon 21-25 PSI ALL30138 Recovery System Radiator Cap Stant Octagon 38-42 PSI