overhead cam

An overhead cam is a camshaft mounted directly above the intake and exhaust valves in the cylinder head of a piston engine, rather than down in the crankcase. The cam lobes push on valve lifters, rocker arms, or valve followers to open and close the valves in precise sequence with crankshaft rotation. This design allows tighter valve timing control and simpler, more compact valve arrangements than shaft-in-block layouts, and it has become the dominant configuration in modern engines.

Two main variants exist: single overhead cam (SOHC), where one shaft operates both inlet and exhaust valves through rocker arms; and dual overhead cam (DOHC), where separate shafts control intake and exhaust valves independently. DOHC engines permit more aggressive cam profiles, variable valve timing without mechanical compromise, and higher maximum engine speeds. The camshaft is typically driven by a toothed belt, chain, or gear train meshed with a sprocket on the crankshaft, maintaining a 2:1 speed ratio so the cam rotates once per two crankshaft revolutions.

Overhead cam engines require valve clearances (the small gaps between cam lobes and lifters or rocker arms) to be set and maintained; excessive clearance causes noise and late valve opening, while insufficient clearance prevents full closing and causes compression loss and overheating. Belt or chain stretch, wear of the cam lobes themselves, and wear of the valve followers or rocker arm pivot points are the primary failure modes. The camshaft must be precisely ground to ensure uniform lobe profiles; even small variations in lobe height or shape damage valve sealing and combustion pressure.

The overhead cam design became standard in passenger cars during the 1960s and 1970s as emissions regulations demanded finer control over valve events. Trucks and industrial engines often retained shaft-in-block designs longer because overhead cams require more sophisticated drive systems, higher manufacturing precision, and better sealing of the cylinder head to avoid oil leakage. Today, even small displacement industrial and agricultural engines commonly use single overhead cams for fuel economy and emissions compliance.

The term itself reflects the physical position: the cam is located overhead relative to the cylinders, visible in the engine bay rather than hidden below. This visibility made the overhead cam a marketing feature when it first appeared and became synonymous with modern, efficient engine design. Technicians and engine builders treat overhead cam engines quite differently from pushrod engines in terms of valve adjustment procedures, failure diagnosis, and rebuild requirements.