Tyre valve stem cap is one of the most overlooked part of a car because this cap is often not regarded as important. But actually it serves a pretty big purpose, it is designed to protect the Schrader valve— a valve stem into which a valve stem core is threaded, that keeps the air or nitrogen in your tyres from leaking.

A valve stem cap is important for the Schrader valve because if one is not fitted on the valve, dirt and moisture can enter the inside of the valve stem, potentially jamming or contaminating the sealing surfaces and causing a leak.

Metal, and some higher quality hard-plastic valve stem caps, often have a rubber washer or seal inside to help make an airtight seal. This type of cap protects from dirt and moisture and also helps prevent air from escaping from a slightly leaking Schrader valve. In addition, the rubber washer or seal prevents the cap from loosening and falling off due to vibrations by acting as a cushion between the cap and the valve stem. Most caps, however, are a softer plastic designed for protection only and are not equipped with a separate seal.

Tyre valve stem caps often get missing because the last time you took your vehicle in for tyre pressure check, it may not have been put back on. It is also possible that the vibrations created by the spinning of your tyres over thousands of miles caused the cap(s) to become loose and fall off.

Always keep in mind that these caps are essential for long lasting tyre valves, so never forget to make sure the caps are on at all times.