IOE engine

The intake/inlet over exhaust, or "IOE" engine, known in the US as F-head, is a four-stroke internal combustion engine whose valvetrain comprises OHV inlet valves within the cylinder head and exhaust side-valves within the engine block.

IOE engines were widely used in early motorcycles, initially with the inlet valve being operated by engine suction instead of a cam-activated valvetrain. When the suction-operated inlet valves reached their limits as engine speeds increased, the manufacturers modified the designs by adding a mechanical valvetrain for the inlet valve. A few automobile manufacturers, including Willys, Rolls-Royce and Humber also made IOE engines for both cars and military vehicles. Rover manufactured inline four and six cylinder engines with a particularly efficient version of the IOE induction system.

A few designs with the reverse system, exhaust over inlet (EOI), have been manufactured, such as the Ford Quadricycle of 1896.