F-head

The intake/inlet over exhaust (IOE) engine, also known as F-head (in the US) and pocket valve, is a valvetrain configuration used in early four-stroke internal combustion engines. The configuration consists of inlet valves located in the cylinder head and exhaust valves located in the cylinder or 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 line 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, or EOI, have been manufactured.

In a F-head/IOE engine, the intake manifold and its valves are located atop the cylinders, in the cylinder head, and are operated by rocker arms which reverse the motion of the pushrods so that the intake valves open downward into the combustion chamber. The exhaust manifold and its valves are located beside or as part of the cylinders, in the block. The exhaust valves are either roughly or exactly parallel with the pistons; their faces point upwards and they are not operated by separate pushrods, but by contact with a camshaft through the tappet or valve lifter and an integrated valve stem/pushrod. The valves were offset to one side, forming what seemed to be a pocket, leading to the term "pocket valve" being used for IOE engines. An F-head engine combines features from both overhead-valve and flathead type engines, the inlet valve operating via pushrod and rocker arm and opening downward like an overhead valve engine, while the exhaust valve is offset from the cylinder and opens upward via an integrated pushrod/valve stem directly actuated by the camshaft, much like the valves in a flathead engine.

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Wikipedia