A dustcap is a small cover used on the valve stem of a bicycle or car tire 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 tire due to slight gas seepage past the tire 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. To prevent corrosion due to electrolysis and malfunction of the electronics in a direct TPMS and avoid costly repairs, metal valve caps without insulating plastic linings should not be used on direct-TPMS-equipped tire valve stems.