Dustcap

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A dustcap is a small cover used on the valve stem of a bicycle or car wheel to prevent dust or other small particles from entering the valve and damaging it. The dustcap also forms a pressure seal, helping to prevent deflation of the tyre due to slight gas seepage past the tyre valve. Dustcaps are usually made from plastic but may occasionally be made of metal. The dustcap is internally threaded and is secured by screwing it onto the end of the stem.

A dustcap may also refer, on a bicycle, to the crankarm bolt cover. It is usually a piece of plastic that, on the crankarm, is snapped or screwed into the threaded counterbore, which houses the bolt that holds the crankarm to its axle, the bottom bracket spindle. The dustcap serves to protect the threads (on the inside of the counterbore), which are used with a crankarm puller to remove the crankarm from the spindle.