When a motorcycle has been ridden for a long time, the engine usually experiences a situation where the valve clearance is either too large or too small. When the valve clearance of the motorcycle is too large or too small, it usually directly affects the performance of the vehicle. By adjusting the valve clearance to the most suitable state, we can enhance the engine performance, making the vehicle more powerful and also reducing the noise of the engine.

01 Cylinder Head Structure

The engine's cylinder head, cylinder block and piston together form the combustion chamber. The cylinder head, due to being exposed to high temperatures and high pressures for a long time, is usually made of aluminum alloy with good cooling effect and high conductivity. This is because its excellent thermal conductivity ensures a better cooling effect.

02 Relationship between Crankshaft and Camshaft

The timing sprocket is installed on the crankshaft, while the camshaft sprocket is mounted on the camshaft of the cylinder head. These two sprockets are linked by the timing chain, and the number of teeth on the camshaft sprocket is twice that of the timing sprocket.

Therefore, in a four-stroke engine, the crankshaft rotates two full circles while the camshaft rotates just one circle. This way, a complete working cycle of intake, compression, combustion and exhaust can be achieved.

03 Camshaft and Valve Rocker Arm

The rotation of the crankshaft is transmitted to the camshaft through the timing chain. The rotation of the camshaft exerts a force on the valves, causing them to open or close. Each rocker arm is installed on the rocker arm shaft. When the camshaft rotates, it pushes the rocker arms upwards, causing them to rotate around the rocker arm shaft. This action opens and closes the valves.

The rocker arm is equipped with a valve clearance adjustment screw. The sliding surface of the camshaft must be adequately lubricated to prevent wear. Therefore, the oil is of vital importance in preventing wear on the cam.

04 Intake Valve Timing

The actual opening and closing times as well as the duration of the intake valves and exhaust valves, corresponding to the crankshaft angle, are referred to as the valve timing.

The valve timing diagram shows the duration during which the intake and exhaust valves open and remain open. From this diagram, it can be seen that only when the intake and exhaust valves are completely closed and the upper ends of the valves are not under the pressure of the rocker arm head, will there be a gap at the upper ends of the intake and exhaust valves.

That is to say: It is not the case that the intake and exhaust valves close completely every time the piston reaches the top dead center. Instead, we should fully utilize the inertia of the airflow to allow the cylinder to fully absorb fresh air and expel the exhaust gases from the combustion process as much as possible.

05 Valve Clearance

There is a gap between the end of the valve stem and the valve adjustment screw, which is called the valve clearance. This gap serves as a space for the valve and related components to expand due to heat, ensuring that the valve is fully closed and sealing the combustion chamber. When the engine is cooled, the valve clearance must be adjusted to the specified range value.

06 Valve clearance variation range

When the engine is started cold, the valves heat up rapidly and expand at a faster rate than the cylinder head. This causes the valve clearance to become smaller than it is at the cold engine state.

When the entire engine is heated, since the cylinder head is made of aluminum, its expansion coefficient under heat is greater than that of the iron valves. The valve clearance increases from small to large. As the engine temperature continues to rise, the valve clearance becomes larger compared to the cold state of the engine.

If the valve clearance setting value is much smaller than the specified value at the cold state of the engine, the rocker arm will continue to press the valve, causing the valve to fail to close completely. This in turn leads to air leakage and unstable idle speed problems.

07 Valve Clearance Adjustment

The valve clearance must be adjusted when the engine is cool. When the camshaft is not pushing the valves and the piston is at the top dead center of the exhaust stroke, since the cam pushes the valves at this time, there is no clearance. Therefore, the valve clearance adjustment cannot be carried out. When adjusting the valve clearance, it is necessary to confirm that the piston is at the top dead center of the compression stroke and to ensure that the T mark on the rotor of the magneto and the mark on the left cover are aligned.

08 Valve Clearance Adjustment Tool

What tools are needed for adjusting the valve clearance?

It is recommended that everyone do their homework before getting started. Only after familiarizing yourself with the repair manual of the engine can you correctly follow the operation procedures. For example, prepare the following tools and equipment:

General tools for disassembling motorcycles

feeler gauge

Micrometer

Torque wrench

Protective gloves

Suction Stick

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