How do you check and adjust the valve clearance of a single-cylinder diesel engine?

The purpose of valve clearance is to ensure that the intake and exhaust valves close tightly and to provide allowance for thermal expansion of the valves and their actuating components. During engine operation, the intake valve is cooled by fresh air and maintains a temperature of 300°C–400°C, whereas the exhaust valve is exposed to high-temperature exhaust gases, resulting in temperatures of 600°C–800°C; consequently, the exhaust valve runs hotter and undergoes greater thermal expansion than the intake valve. Therefore, the exhaust valve clearance is generally set larger than the intake valve clearance. However, in some diesel engines, the intake and exhaust valve clearances are set equally because the exhaust valve is made of a material with a lower coefficient of thermal expansion or features enhanced heat dissipation.

Valve clearance is defined as the gap between the end of the valve stem and the rocker arm when the valve is fully closed.

During the service life of a diesel engine, valve clearance can change due to factors such as component wear, loosening of adjustment screws, or the removal, reinstallation, and tightening of cylinder head nuts. If the clearance is too small, thermal expansion causes the components to elongate, preventing the valves from closing tightly and resulting in a loss of engine power; additionally, high-temperature cylinder gases may leak through the gap, causing the valves to overheat or even burn out. If the clearance is too large, the impact between the valve and the valve seat intensifies, shortening service life; furthermore, the valve opening duration is reduced, hindering the intake of fresh air and the expulsion of exhaust gases, which leads to a drop in engine power. Therefore, regular inspection and adjustment of valve clearance are essential to ensure the diesel engine operates correctly. The inspection and adjustment procedures are as follows:

(1) Remove the cylinder head cover while the diesel engine is cold;

(2) Rotate the flywheel to align the Top Dead Center (TDC) mark on the flywheel with the pointer mark on the housing, positioning the piston at TDC on the compression stroke;

(3) Use a feeler gauge to measure the clearance between the end of the valve stem and the rocker arm; if the feeler gauge cannot be inserted or if there is still excessive clearance after insertion, the valve clearance must be adjusted.

(4) Loosen the lock nut on the valve clearance adjustment screw, turn the adjustment screw while measuring with the feeler gauge until the measured value matches the specified value, and then tighten the lock nut while holding the adjustment screw stationary.

(5) After adjusting the intake and exhaust valve clearances, rotate the crankshaft several times and re-measure the clearances; if any change is detected, readjustment is required.