Valve Cap
A small threaded plastic or metal cap screwed onto the end of a valve stem to keep out dust and moisture.
A valve cap is a small, threaded cap that screws onto the tip of the tire valve stem. While many drivers believe the cap is what keeps the air inside the tire, its primary function is actually to protect the delicate valve core inside the stem from dirt, dust, mud, water, and road salt.
If road debris gets trapped inside the valve stem, it can damage the valve core's rubber seal or cause the pin to stick open, leading to a slow and persistent air leak. Additionally, water entering an uncapped valve stem can freeze in winter, pushing the valve core open and causing sudden tire deflation.
Valve caps are typically made of black plastic, but metallic versions are also common. When using metal caps on aluminum valve stems, it is important to ensure they are nickel-plated or have a plastic liner to prevent galvanic corrosion, which can fuse the cap to the stem.