Why does the valve clearance automatically become larger or smaller?

The valve clearance values of several diesel engines are shown in Table 3-6.

Table 3-6 Valve Clearance Values of Several Models (mm)

After adjusting some valve clearance, the working time is very short, even less than a few hours, and the valve clearance changes obviously, some become larger and some become smaller. The reasons for the rapid increase of valve clearance may be:

(1) The gap between the rocker shaft and the rocker sleeve is too large.

(2) The fastening screw of the rocker shaft and the valve clearance adjustment screw are loose or slipping.

(3) It is also possible that the valve guide does not go down to the specified position, causing the push rod to bend or the top of the rocker arm to crack.

The rapid decrease of the valve clearance indicates that the running-in surface of the valve is wearing at high speed, which may be due to the abrasive grinding the valve and the air is not cleaned or filtered, or the material of the cylinder head is too soft (meaning that the valve seat is not embedded).