Presta valve
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The Presta valve is a valve (commonly) found in high pressure road style and higher end mountain bicycle tires, although it is now used on all UST or "tubeless" mountain bike tires. It consists of three major components: the valve body, a threaded valve stem, and a lock nut. In order to accommodate different rim wall depths, the valve body is available in multiple lengths. Like the Schrader valve, air pressure inside the tire holds the valve shut; however, unlike the Schrader, the Presta has no return spring to help keep the valve shut. Instead, a threaded stem extends out of the valve core to hold a locking nut that can be screwed down to ensure the valve remains closed.
In addition, the Presta valve has a much smaller diameter than the Schrader valve. This helps the strength of wheels with narrow rims because the weakest point on such rims is the location of the hole for the valve. A Presta valve requires a different chuck on the bicycle tire pump than the larger Schrader valve; simple threaded brass adapters are available to allow a Presta valve tube to be inflated by a pump or air chuck designed for Schrader valves.
The Presta valve must be manually unscrewed to permit airflow in either direction. The screw, located at the tip of the valve, remains within the valve housing even after it is unscrewed; unscrewing the valve permits the plug to move up and down within the housing. It must be unscrewed before attaching a tire pump. Once the tire pump is removed, the valve should be screwed down to prevent accidental air leakage. A screw cap provides protection to the valve top, although the cap is not necessary to keep air from leaking.
