Single overhead camshaft is a design in which one camshaft is placed within the cylinder head. In an inline engine this means there is one camshaft in the head, while in a V engine there are two camshafts: one per cylinder bank.
The SOHC design has less reciprocating mass than a comparable pushrod design. This can allow for higher engine speeds, which in turn will increase power output for a given torque. The cam operates the valves directly or through a rocker arm as opposed to overhead valve pushrod engines, which have tappets, long pushrods and rocker arms to transfer the movement of the lobes on the camshaft in the engine block to the valves in the cylinder head.
SOHC designs offer reduced complexity compared to pushrod designs when used for multivalve heads, in which each cylinder has more than two valves. An example of a SOHC design using shim and bucket valve adjustment was the engine installed in the Hillman Imp,(4 cylinder, 8 valve) a small early 1960's 2 door saloon car with a rear mounted alloy engine based on the 'Coventry Climax' FWMA race engines. Exhaust and inlet manifolds were both on the same side of the engine block, (thus not a crossflow cylinder head design).However this did offer excellent access to the spark plugs!
In the early 1980's Toyota and Volkswagen also used a directly actuated, SOHC parallel valve configuration with two valves for each cylinder. The Toyota system used hydraulic tappets while the Volkswagen system used bucket tappets with shims for valve lash adjustment. Of all valvetrain systems this is the least complex configuration possible.