HKT RADIATOR CAPS C-21 LARGE MADE IN JAPAN

A radiator cap acts as a valve by adjusting the pressure within the radiator.
When the temperature of coolant water increases, the pressure within the radiator increases, and when the temperature of coolant water decreases, the pressure within the radiator decreases.
When the pressure within the radiator increases because the temperature increases, the main pressure valve starts working to adjust the pressure.
When the pressure is reduced, the negative pressure valve starts working to keep the pressure at a proper level.