For maximum economy and performance it is important that the valve clearances are cor- rect; most manufacturers recommend that they are checked at regular service intervals, usually every 5,000—6,000 miles. Some engines have hydraulic tap- pets which automatically pro- vide the correct setting, and these do not need checking. Some engines need special tools to check the valve clear- ances, and in this instance valve checking and adjustment should be left to a garage.

Hot and cold adjustment

The car manufacturerwill indi- cate in the handbook or work- shop manual whether the valve clearances are to be checked and adjusted with the engine hot or cold. For practical pur- poses, a cold engine is one that has stood idle overnight, whereas a hot engine is one at normal operating temperature, which is achieved after driving at least four miles. On afew engines the manufacturer will recommend a specific engine temperature for checking and adjusting valve clearances. If this is the case the job is best left to professionals.

Making adjustments

The direct-acting overhead camshaft system is the easiest to check, but can be the har- destto adjust, because on some engines the camshaft must come out and the bucket tappets lifted so that adjusting shims can be fitted underneath —a major dismantling job. To keep servicing costs down, manufacturers of these engines may specify a minimum valve clearance. Pro- vided the gap does not go below this figure, the engine will operate satisfactorily. On most other valve mechanisms, adjustment of the clearances is made by turning an adjusting screw or nut and is straight- forward.

Inlet and exhaust clearances

Many engines have a different clearance figure quoted for inlet and exhaust valves. Since all valves look the same when viewed from the stem end, they can be distinguished by tracing the line of the two manifolds.

The inlet manifold branches will aim at the inlet valves, and the branches of the exhaust manifold will point towards the exhaust valves.

Turning the engine

Because each valve clearance must be checked with the valve in the fu//yclosed position, it will be necessary to turn the engine to check them all. On in-line engines it is possible to ‘pair’ the valves and save a lot of engine turning. On four- cylinder in-line engines the ‘rule of nine’ is used. Number the valves from the generator end. Turn the engine until one valve is fully open. Subtract its number from nine, and the ans- wer is the valve to check. For instance, when valve no.2 is fully open, 9—2=7, so valve no. 7 is the one to check. The same principle applies to six-cylin- der in-line engines, using the ‘rule of 13’. The crankshaft can be turned by using a spanner on the crankshaft pulley nut.

Using a feeler gauge

If the clearances are incorrect, on most &ngines it is only a matter of turning an adjusting screw to put each one right. The clearance is correct when a clean feeler will slide into the gap with only moderate end pressure. If the gauge is a loose fit the gap is too wide. If the blade buckles under pressure, it is too small.