Why should the automobile engine adjust the valve clearance?

The purpose of adjusting valve clearance is to keep the expansion coefficient of valve metal. Its function is to prevent the valve metal from expanding without expansion gap, which leads to the leakage of the valve due to lax closure. Excessive valve clearance will produce metal knocking sound, while too small valve clearance will lead to lax closing and air leakage.

When the engine is working, the valve will expand due to the increase of temperature. If there is no gap between the valve and its transmission parts in the cold state or the gap is too small, then the thermal expansion of the valve and its transmission parts in the hot state will inevitably lead to the untidy closing of the valve, resulting in air leakage in the compression stroke and power stroke of the engine, thus reducing the power and even making it difficult to start. In order to eliminate this phenomenon, when the engine is assembled in cold state, a certain gap is usually left in the valve and its transmission mechanism to compensate for the expansion of the valve after heating. This clearance is called valve clearance.