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On most modern overhead cam engines, the camshaft is directly over the valves, as pictured. In some other overhead cam engines, a rocker arm pivots on a fixed shaft while one projection of the rocker-arm rides on a cam of the rotating camshaft. In both cases, this creates an oscillating linear motion, opening the valve. The closing of the valve is typically accomplished by a compression spring placed between the valve collet (or retainer) and the cylinder head above the combustion chamber. In overhead valve engines, the camshaft is located near the crankshaft and motion of the cam lobes is followed by cam followers (similar to tappets) and transferred up to the cylinder head and the rocker arm assembly by means of long pushrods.
Read more about this topic: Tappet