Overhead valve engine

An overhead valve engine (OHV engine) is an engine in which the valves are placed over the cylinder head. This was an improvement over the older flathead engine, where the valves were placed in the block next to the piston. Overhead camshaft (OHC) engines, while still overhead valve by definition, are usually categorized apart from other OHV engines.

In a piston engine configuration where the valves are overhead but the camshaft is not, informally called pushrod engine or I-head engine, the camshaft is placed within the cylinder block (usually beside and slightly above the crankshaft in a straight engine or directly above the crankshaft in the V of a V engine), and uses pushrods or rods to actuate rocker arms above the cylinder head to actuate the valves. Lifters or tappets are located in the engine block between the camshaft and pushrods. By contrast, overhead camshaft design avoids the use of pushrods by putting the camshaft directly above the valves in the cylinder head, thus simplifying the valvetrain.