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Schrader and Dunlop valve stems are 8 mm in diameter, whereas Presta valve stems are 6 mm, allowing Prestas to be used on narrower, high-performance rims as on road racing bicycles. Another disadvantage of the Schrader is that debris may be introduced into its spring-loaded pin, impairing inflation, whereas the Presta valve relies only on air ...
To reduce friction, uneven wear and "bell-mouthing" of the valve guide, [1] a roller rocker uses needle bearings (or a single bearing ball in older engines) at the contact point between the rocker and the valve. These allow higher engine speeds (RPMs) and higher loads, and were initially confined to high-performance and racing engines due to ...
Exhaust valves are subject to very high temperatures and in extreme high performance applications may be sodium cooled. The valve is hollow and filled with sodium, which melts at a relatively low temperature and, in its liquid state, convects heat away from the hot valve head to the stem where it may be conducted to the cylinder head.
A tappet or valve lifter is a valve train component which converts rotational motion into linear motion in activating a valve. It is most commonly found in internal combustion engines , where it converts the rotational motion of the camshaft into linear motion of intake and exhaust valves, either directly or indirectly.
Powder metallurgy materials are also commonly used in the manufacture of valve guides, which have the advantages of flexible material composition and the self-lubricating properties. [2] The clearance between the inner diameter of the valve guide and the outer diameter of the poppet valve stem is critical for the proper performance of an engine.
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.