The so-called valve clearance refers to: when the engine is in a cold state, the valve transmission mechanism leaves a certain gap. To compensate the expansion of valve and transmission mechanism after heating.

The valve will expand as the engine works. 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 expansion potential of the valve and its transmission parts will automatically open the valve and cause the valve to close loosely, resulting in the leakage of the engine in the compression and work stroke, and the power decline, serious so that the engine is not even easy to start. In order to eliminate the above phenomenon, usually when the engine is assembled in cold state, a proper gap is left in the valve and its transmission mechanism to compensate for the expansion of the valve after heating.

Valve clearance depends on the overall structure of the valve mechanism, and this clearance can also be adjusted. The size of valve clearance is generally determined by the engine manufacturer according to the test. Usually in cold condition, the clearance of intake valve is 0.25~0.30mm, and the clearance of exhaust valve is 0.3~0.35mm. If the clearance is too small, the engine may leak in the hot state, leading to a serious reduction in power, will make the valve burned out. If the clearance is too large, impact and sound will occur between the transmission parts and between the valve and the valve seat, and accelerate the wear, but also reduce the duration of the valve opening,

With hydraulic tappet engines, the length of the tappet can be automatically changed to compensate for the thermal expansion of the valve at any time, so there is no need to reserve valve clearance. Such as FAW Audi, Santana cars do not need to reserve valve clearance.