Flathead engine

A flathead engine (a.k.a. sidevalve engine (SV), flathead, 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, other names such as L-block or L-head are also common.

The sidevalve'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, most commonly without any further valvetrain components (such as pushrods, rocker arms, overhead valves or overhead camshafts). Indian, however, employs both rocker arms and pushrods to transmiit motion from the cam lobes to the valve stems.

The sidevalve arrangement was once the most common across all motor industries (automotive, agricultural, marine, aviation, and others), but it has since fallen from favor in most multicylinder applications, such as automotive and aviation, having been displaced by overhead valve designs. Flathead designs are still commonly built new for many small engine applications of one and two cylinders, such as lawnmowers, rotary tillers, and two-wheel tractors.

The main advantages of a sidevalve engine are simplicity, reliability, low cost, compactness, responsive low-speed power, low mechanical engine noise and insensitivity to low-octane fuel. The absence of a complicated overhead valve (OHV) valvetrain allows a compact engine that is less expensive to manufacture, as the cylinder head may be little more than a simple metal casting. These advantages may explain why OHV designs were first adopted only in high-performance applications such as aircraft, luxury cars, and sports cars, while economy cars, trucks, and agricultural engines continued to use flathead engines for some time. Even today, flatheads are common in lawnmowers and basic farm machinery.

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