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A valve stem is a self-contained valve that opens to admit gas to a chamber (such as air to inflate a tire), [1] and is then automatically closed and kept sealed by the pressure in the chamber, or a spring, [2] [3] or both, to prevent the gas from escaping.
The original Schrader valve design was invented in 1891, and patented in the United States in 1893. [2] The Schrader valve consists of a valve stem into which a valve core is threaded. The valve core is a poppet valve assisted by a spring. A small rubber seal located on the core keeps the fluid from escaping through the threads.
However, these generally have the roller at the point where the cam lobe contacts the rocker, rather than at the point where the rocker contacts the valve stem. Friction may be reduced at the point of contact with the valve stem by a roller tip. A similar arrangement transfers the motion via another roller tip to a second rocker arm.
[5] [6] Replacement of the engine's water pump at the same time is often recommended, since the water pump is also subject to wear and easily accessed during the replacement of the timing belt. Timing belts are typically located in front of the engine and are often behind a cover for protection against dust and debris.
The Presta valve (also French valve (FV) or Sclaverand valve) is a type of tire valve commonly found on high pressure bicycle inner tubes and is commonly used on tubeless setups. It consists of 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.
The inlet camshaft position is advanced 25 degrees, thus increasing valve overlap. It remains in this advanced state until about 5000 rpm when the solenoid switches off, and the variator piston returns the valve timing to its natural state. The variable timing increases the engine's mid-range flexibility and reduces emissions.