Most four-stroke engines use two poppet valves to allow the mixture into the cylinder and let the exhaust out.

As they are in direct contact with the combustion process, both valves are made from heat-resistant material.

Of the two, the inlet valve, which is cooled by the inrush of petrol/air mixture on each induction stroke, runs cooler. The exhaust valve, which in normal use runs at a red-hot 80000, is usually made of a higher-temperature alloy steel than the inlet, and transfers much of its heat to the cylinder head when it is closed.

Both valves are shut by a powerful spring, usually in the form of a coil round the outside of the valve stem. The bottom of the spring rests on the cylinder head casting and the top presses against a retainer fitted to the end of the valve stem. Some valves have two coil springs, fitted one inside the other, to shut them.

A valve is opened simply by being pushed down against spring pressure, and there are several methods of doing this.

Valve adjustment

Metal expands when it is heated, and to make sure the valves are able to shut fully when the engine is hot, a small amount of play or clearance is whether the clearance should be checked with the engine hot or cold. It is important thaUhe recommended clearances and checking system are used, since incorrect adjustment can valve gear.

The clearance reduces as to expensive overhaul, screw clockwise reduces it. face of the biscuit’ the engine gets hot, but should never disappear altogether. On pushrod and rocker layouts where the camshaft is a long way from the valve, a larger clearance is needed than on a direct-acting overhead cam- shaft system. The valve clear- ance is measured when the valve is fully shut by checking the slack in the operating un- kage with a feeler gauge. The manufacturer will specify.

A few engines have hydraulic tappets which have two patts, one sliding within the other. Oil under pressure expands these tappets and takes up the clearances when the engine is running. On these engines, no valve adjustment is needed.

Driving the camshaft

Until the development of the internally-toothed rubber belt, most overhead camshafts were chain-driven. Because of the length of chain involved, a ten- sioner was needed to prevent whipping. The tensionerwas in the form of either a synthetic rubber pad, spring-loaded or hydraulically pressed against the side of the chain, or a spring blade or rubber-faced steel strip bearing on one side of the chain.

A toothed-belt camshaft drive is quieter than a chain and, since it needs no lubrica- tion, it can be mounted exter- nally. The oil-resistant rubber is moulded on to non-stretch cords and on the inside has a series of square-section teeth that accurately match cut-outs in the crankshaft and camshaft pulley wheels. The belt is usu- ally tensioned by a jockey wheel which bears on its smooth side.