<P> The stem is used to push down on the valve to open it, with a spring generally being used to return it to the closed position when the stem is not being depressed . At high revolutions per minute (RPM), the inertia of the spring means it cannot respond quickly enough to return the valve to its seat between cycles, leading to' valve float' . In this situation desmodromic valves can be used which, being closed by a positive mechanical action instead of by a spring, are able to cycle at the high speeds required in, for instance, motorcycle and auto racing engines . </P> <P> The engine normally operates the valves by pushing on the stems with cams and cam followers . The shape and position of the cam determines the valve lift and when and how quickly (or slowly) the valve is opened . The cams are normally placed on a fixed camshaft which is then geared to the crankshaft, running at half crankshaft speed in a four - stroke engine . On high - performance engines, the camshaft is movable and the cams have a varying height so, by axially moving the camshaft in relation with the engine RPM, the valve lift also varies . See variable valve timing . </P> <P> For certain applications the valve stem and disk are made of different steel alloys, or the valve stem may be hollow and filled with sodium to improve heat transport and transfer . Although a better heat conductor, an aluminium cylinder head requires steel valve seat inserts, where a cast iron cylinder head would often have employed integral valve seats in the past . Because the valve stem extends into lubrication in the cam chamber, it must be sealed against blow - by to prevent cylinder gases from escaping into the crankcase, even though the stem to valve clearance is very small, typically 0.04 - 0.06 mm, so a rubber lip - type seal is used to ensure that excessive oil is not drawn in from the crankcase on the induction stroke, and that exhaust gas does not enter the crankcase on the exhaust stroke . Worn valve guides and / or defective oil seals can often be diagnosed by a puff of blue smoke from the exhaust pipe on releasing the accelerator pedal after allowing the engine to overrun, when there is high manifold vacuum . Such a condition occurs when changing gear . </P> <P> In multi-valve engines, the conventional two - valves - per - cylinder setup is complemented by a minimum of an extra intake valve (three - valve cylinder head) or, more commonly, with an extra intake and an extra exhaust valve (four - valve cylinder head), the latter meaning higher RPM are, theoretically, achievable . Five valve designs (with three inlet and two exhaust valves) are also in use . More valves per cylinder means improved gas flow and smaller reciprocating masses may be achieved, leading to improved engine efficiency and, ultimately, higher power output and better fuel economy . </P>

In conventional valve which component is used to move the spool