A flathead engine (aka side valve engine, or flatty) is an internal combustion engine with valves placed in the engine block beside the piston, instead of in the cylinder head, as in an overhead valve engine. As the cylinder cross-section has the shape of an upside-down L, this leads to other names:
L-block, or L-head.
The side valve's poppet valves are usually sited on one side of the cylinder(s). A recess in the cylinder head creates a corridor connecting the valves and the combustion chamber. The valve gear comprises a camshaft which operates the valves via simple tappets, without any further valve train paraphernalia (such as pushrods, rocker arms, overhead valves or overhead camshafts). The side valve arrangement was common in early engine designs, but it has since fallen from favour.