A flathead engine, also known as a sidevalve engine or valve-in-block engine is an internal combustion engine with its poppet valves contained within the engine block, instead of in the cylinder head, as in an overhead valve engine. Flatheads were widely used internationally by automobile manufactures from the late 1890's until the mid-1950's but were replaced by more efficient overhead valve and overhead camshaft engines. They are currently experiencing a revival in low-revving aero-engines such as the D-Motor.