What are radiator caps and why do you need them? A radiator cap is a small but vital part of your radiator, sealing the opening of radiator, keeping the system closed and preventing air from getting into the system. In essence, its primary function is to ensure the system works efficiently. It can also improve the aesthetics of your radiator and, in turn, the look of your space. Our range of radiator caps offers style and functionality in abundance, so, why not pick the perfect radiator cap from the selection below?