overhead valve
A type of valve in an internal combustion engine, and thus also (metonymic) the type of valvetrain design, the type of engine that uses it, or an instance of such an engine; this design has the inlet and exhaust valves in the cylinder head (and therefore above the piston). The camshaft can be beside the pistons (with pushrods and rockers operating the valves) or above the valves, with the latter usually referred to as overhead cam.
overhead valve: valves mounted in the cylinder head, not the block
An overhead valve engine positions its inlet and exhaust valves in the cylinder head, directly above the combustion chamber, rather than in the engine block beside the pistons. This arrangement improves gas flow into and out of the cylinder, allowing larger valve diameters and better breathing at higher RPMs. The design became dominant in automotive and small engine manufacture because it delivers more power and efficiency than the older sidevalve or flathead layout.
Two mechanical arrangements exist within overhead valve design. In pushrod engines, a camshaft mounted low in the block operates pushrods that lift rocker arms, which in turn open the valves in the head. In overhead cam engines, the camshaft sits directly in or above the cylinder head, eliminating the intermediate linkage. Overhead cam (OHC) engines reduce reciprocating mass and can operate at higher RPMs with less wear, but pushrod designs remain cheaper to manufacture and are simpler to service in many truck and industrial applications.
Common problems and maintenance
Valve seats in the cylinder head wear and can become pitted, requiring periodic grinding or replacement to maintain a good seal. Valve stems slide through guides that eventually wear, causing oil to leak past the seals and increasing blow-by. Pushrod engines suffer additional wear in the rocker arms and valve lifters as the metal-to-metal contact surfaces accumulate microscopic damage over thousands of operating hours. OHC engines avoid some of these wear modes but require more precise timing belt or chain adjustment.
The term "overhead valve" originated in the 1910s as manufacturers experimented with valve placement. Early engines mounted valves alongside cylinders to simplify casting and machining, but overhead placement allowed hemispherical or wedge-shaped combustion chambers that burn fuel more efficiently. The name reflects the physical location: the valves are overhead, in the head, as opposed to flat on the sides. Modern engines almost universally use overhead valves because the alternatives offer no practical advantages in passenger cars or light industrial equipment.