A valve is one engine mechanism used in four-stroke engines.
A valve on the side where the air-fuel mixture or air is drawn in is referred to as an intake (inlet) valve, while a valve on the side where exhaust gas is discharged is referred to as an exhaust valve.
One valve is provided per cylinder in small engines, while three to four of them are provided per cylinder in large engines.
Furthermore, to reduce intake valve intake resistance, intake valves typically have larger valve diameters than exhaust valves.
Valve shape is that of a mushroom valve (poppet valve) as shown in the figure to the right, with the valve top part being referred to as a valve head and the valve bar part being referred to as a valve stem.
Furthermore, the part that makes contact with the cylinder head valve seat is referred to as the valve face, and is fashioned into a conical shape to fit tightly with the valve seat and maintain airtightness.
Since valves are exposed to high temperatures and high pressures while engines are running, they are thus required to:
(1) Withstand high temperatures and have good thermal conductivity (cooling properties);
(2) Not corrode when exposed to high temperature gas; and
(3) Be highly wear resistant (durability).
Therefore, they are made of special heat resistant steels such as nickel chrome steel, cobalt chrome steel, and tungsten steel. Valve heads undergo heat resistance treatments, and valve stems are subject to high frequency induction hardening.

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