The Presta Valve (also called Sclaverand valve or French valve) is a valve commonly found in high pressure road style and many mountain bicycle inner tubes and tubeless systems. It comprises an outer valve stem and an inner valve body. A lock nut to secure the stem at the wheel rim and a valve cap may also be present.
Unlike Schrader cores, the inner parts of the valve, the valve core, in most cases cannot be removed from the stem. Some Presta products allow core removal but they are still quite rare. This has consequences for those who intend to add puncture repair fluids to the inner-tube; the usual way is to remove the valve core before pouring in the substance. Such users are reduced to buying ready-filled tubes or deliberately puncturing tubes for the filling process.
The outer valve stem is manufactured in various lengths for different applications, and has a narrower diameter (6 mm) than the Schrader valve (8 mm), which helps strengthen narrow rims because the weakest point of a bicycle rim is usually the valve hole.
The air pressure in an inflated tire holds the inner valve body shut. A small screw and captive nut on the top of the valve body permits the valve to be screwed shut and ensure that it remains tightly closed. The nut must be unscrewed to permit airflow in either direction (this must be done before attaching a pump). The screw remains captive on the valve body even when unscrewed fully; it is tightened again after the tire is inflated and the pump removed.
The valve cap protects the valve body, and also prevents the valve from damaging the tube when it is rolled for storage, but is not necessary to prevent pressure loss. Because the rims of bicycles drilled for Presta valves cannot accommodate the wider Schrader valves, it is often the case that rims need to be drilled for such replacements. Conversely, when a Presta Valve is fitted into the larger Schrader rim hole, grommets or reducers are sometimes used to take up the extra space.
An advantage of the Presta Valve, in addition to the smaller hole required in narrow rims, which preserves rim strength, is that there is no return spring, which allows for easier hand pumping. Opinions vary on which is the better valve type; the Schrader is far more prevalent in automotive applications and most compressor fittings are designed for Schrader valves.
The standard Presta valve has an external thread. An adaptor can be fitted onto this external thread to permit the Presta valve to be connected to a pump with a Schrader chuck. The same adaptor,because of a coincidence of thread sizes, can convert a direct-fitting Schrader pump into one that can connect to flexible adaptors of either kind.