Why set the valve clearance? What's the harm if the valve clearance is too large or too small?

The purpose of setting the valve clearance is to ensure the normal operation of the valve mechanism of internal combustion engine. Because the valve mechanism is in a high-speed state and the temperature is high, the valve will expand due to the temperature increase.

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 avoid this phenomenon, when designing the valve train, there is a certain gap between the tail end of the intake and exhaust valve rod and the adjusting screw on the tappet (or rocker arm) to compensate the expansion of the valve after heating.

Extended data:

The size of engine valve clearance varies with different models. Generally speaking, the gap between intake valve and exhaust valve is about 0.25mm~0.35mm, and the gap between exhaust valve and exhaust valve is 0.30 mm ~ 0.40 mm ... Because the exhaust valve is heated more when working, its deformation is also larger, so the gap between exhaust valve is usually larger than that of intake valve.

Valve clearance is an important parameter in the process of automobile design and manufacture, which has been set at the factory, and it can not be too large or too small.

Because the valve and its transmission mechanism will wear in the long-term working process, the valve clearance will also change, which requires regular adjustment of the valve clearance. If it cannot be adjusted in time, it will affect the performance of the engine.