The role of the valve is to be responsible for inputting air into the engine and exhausting the exhaust gas after combustion. From the engine structure, it is divided into intake valve and exhaust valve. The role of the intake valve is to draw air into the engine and mix it with fuel for combustion; the role of the exhaust valve is to exhaust the exhaust gas after combustion and dissipate heat.

When the engine is operating, the valve will expand as the temperature rises. If there is no gap between the valve and its transmission parts in the cold state or the gap is too small, then in the hot state, the heated expansion of the valve and its transmission parts will inevitably cause the valve to close tightly, causing the engine to leak during the compression stroke and the power stroke Gas, so that the power drops, even in severe cases, it is not easy to start. In order to eliminate this phenomenon, when the engine is coldly assembled, a certain gap is left in the valve and its transmission mechanism to compensate for the expansion of the valve after heating. This gap is called the valve gap.