The Morris Minor has a large radiator in relationship to the size and typical perforamnce demands put on the motor. But even a generous size radiator will not be up to the task unless the radiator cap is in good working order. A good cap is designed to keep the water under just the right amount of pressure to keep it contained but not so much pressure to cause the radiator to burst. The cap reach required for the Morris radiator means your options are few. The best and most common option is this 4 LB cap.