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Checking valve clearances

Why is a clearance necessary?

Valves must close in order to seal the combustion chamber completely during the engine’s compression stroke. Therefore it is important that the rocker arm does not contact the end of the valve stem at this precise point. Creating a small gap (clearance) in the region of 0.08 mm between the rocker arm and valve stem ensures the valve face can seal fully against its seat.

Over a period of time wear will take place between the rocker and the end of the valve stem and between the valve seating surfaces in the cylinder head and the valve face itself. It is necessary to check and adjust the valve clearance to maintain the correct clearance and ensure optimum performance from the engine. Excessively large clearances will also be fairly audible as valve clattering.

The motorcycle manufacturer will specify a period at which the valve clearances should be checked. Typically this is every 3000 miles. They will also recommend a specific clearance, sometimes this differs between the intake and exhaust valves. Because the exhaust valve runs at a higher temperature than the intake, which has relatively cool incoming mixture passing around it, the clearances for exhaust valves are generally slightly wider to account for the greater expansion of the components.

What tools do you need?

A set of blade type feeler gauges is essential. Small open-end spanners to fit the valve adjuster can be obtained, but the job can easily be done with a pair of long-nose pliers (photo 1).

How to measure the valve clearance

1. Valve clearances are always measured with the engine cold. This is because the components in an engine which has just been run will have been affected by heat expansion and the clearances will be different than when cold.

2. Remove the spark plug – both plugs if working on a twin (photo 2). This will make the task of turning the engine over easier.

3. Where the engine has individual access caps, remove these from the front and back of the cylinder head (photo 3). Where no caps are provided, remove the valve cover (photo 4).

4. The clearance for each valve is measured when the valve is closed. To open and close the valves the engine must be turned. To turn the engine, place the motorcycle on the centrestand so that the rear wheel is off the ground, select a high gear, and rotate the rear wheel by hand in its normal direction of rotation while you watch the action of the rockers on the valves.

5. Turn the rear wheel until the end of the rocker above the valve being checked is at its highest point and the valve is closed – you should be able to feel some freeplay between them by moving the rocker with your fingers (photo 5).

6. Insert a feeler gauge of the same thickness as the correct valve clearance (see below) in the gap between the rocker arm and the valve stem (photo 6). The intake valve(s) are on the back of the cylinder head and the exhaust valve(s) are on the front. The gauge should be a firm sliding fit – you should feel a slight drag when you pull the gauge out.

7. If the gap (clearance) is either too wide or too narrow, slacken the locknut on the adjuster in the rocker arm (photo 7). Turn the adjuster as required using a screwdriver if the adjuster is slotted, or a very small spanner or a pair of pliers if not, until the gap is correct and the feeler gauge is a sliding fit. Hold the adjuster still and tighten the locknut. Recheck the clearance after tightening the locknut.

8. When all clearances are correct, either fit the access caps (photo 8), or install the valve cover. Make sure the gasket is in good condition and fit a new one if necessary.

9. Install the spark plug(s) (photo 9).

Valve clearances

Those recommended for the range of Chinese 125cc models covered in this new Haynes manual are:

OHV single cylinder engine and OHC twin cylinder engines, set the clearance for intake and exhaust valves to 0.08mm.

OHC single cylinder engine, set the clearance for the intake valve to 0.08mm and the exhaust valve to 0.15mm.