The valve clearance is set to ensure the normal operation of the valve mechanism in a four-stroke motorcycle. Since the valve mechanism operates at high speed and at high temperatures, if the parts such as the valve tappet and the valve rod are heated and expand, they will automatically push open the valve, causing the valve and the valve seat to not close tightly, resulting in air leakage. To prevent this from happening, when designing the valve mechanism, a certain gap is left between the adjusting screws at the ends of the intake and exhaust valve rods and the tappet (or rocker arm). This gap is the valve clearance.

Please provide the text that needs to be translated accurately and fluently into

1. Various Valve Forms of the Valve Train in Four-Stroke Engines The valve forms of the valve train in a four-stroke engine vary depending on the position of the valves. They include side-mounted valves (SV), overhead valves (OHV), overhead camshaft valves (OHC), and double overhead camshaft valves (DOHC).

Structurally, the side-mounted valves are the simplest. However, when this valve type is adopted, the engine's anti-explosion performance and high-speed performance are poor. It can only be used in engines with low compression ratios and low rotational speeds. This valve type has been abandoned in foreign countries. Currently, in China, only two models, the Changjiang 750 and Shandong 750, use this valve type. In terms of performance, the overhead camshaft valve type is the most ideal. It can appropriately increase the rotational speed, high compression ratio, and power requirements for high-power vehicles, while also having good economy. Therefore, it has been widely applied. In recent years, Chinese-produced models such as Jin Cheng CJ70, JC70, Jialing JH70, Shuang狮 90， and those imported from Japan such as CG125 all adopt this valve type. The top-mounted valve structure is more complex. Currently， it is still used in the United States， former West Germany (in the R series motorcycles produced by BMW factories)， and Italy and other countries due to their production habits.

II. Adjustment of Valve Clearance

When a vehicle is in use, due to the wear of the parts in the valve mechanism or the loosening of the adjustment screws, the valve clearance will change. Therefore, regular inspections and adjustments must be carried out.

The adjustment method for valve clearance in the overhead camshaft type engine

(1) Remove the intake and exhaust valve cover and the outer cover of the magneto.

(2) Rotate the rotor of the magnetic motor so that the "T" marks on the outer surface of the rotor align with the marks on the casing. At this point, the piston should be at the top dead center of the compression stroke.

(3) Carefully insert a feeler gauge with a thickness equal to the specified valve clearance into the valve gap and pull it back and forth. If you feel a slight resistance, it indicates that the clearance is appropriate.

(4) If the gap is not suitable, adjust the nut. Use a small wrench to rotate the adjusting screw while pulling the feeler gauge to check the gap. Once the gap is appropriate, tighten it. If the gap changes after tightening, re-measure it with the feeler gauge.

2. Adjustment method for side-mounted valve clearance

Remove the valve cover and remove the spark plugs. Cover the spark plug holes with your fingers, then step on the start lever. When you feel a gust of air hitting your fingers, it indicates that the piston is in the compression stroke. At this point, insert the head of the screwdriver into the spark plug hole and slowly step on the start lever again. When the screwdriver reaches its highest point, the piston is at the top dead center. Check and adjust the valve clearance. The method is the same as that for adjusting the valve clearance of a top-mounted camshaft.

3. Several Issues to Be Clarified

For side-mounted valves, the valve clearance refers to the gap between the tail end of the intake and exhaust valve rods and the adjusting screw on the tappet; for overhead valves and overhead camshaft type valves, the gap is between the top of the valve rod and the adjusting screw on the rocker arm.

There are two types of valve clearance: cold clearance and hot clearance. The hot clearance is slightly smaller than the cold clearance. The clearance measured when the engine is cold is the cold clearance. Generally, the cold clearance of the intake valve is between 0.08 and 0.10 millimeters, and that of the exhaust valve is around 0.10 to 0.12. When adjusting, it is necessary to strictly follow the clearance specified in the manual. The inspection and adjustment of valve clearance must be carried out when the intake and exhaust valves are completely closed. When the piston is at the top dead center of the compression stroke, the intake and exhaust valves are exactly completely closed, so when adjusting the valve clearance, the piston must be at the top dead center.

The adjustment of valve clearance should be carried out when the engine is cold. It is strictly forbidden to make the adjustment while the engine is running. For two-cylinder engines, the inspection and adjustment should be conducted cylinder by cylinder.
(The text is from the Internet. If there is any infringement, please contact for deletion.)