Maisto 1:18 1952 Harley Davidson K Model, White

The Harley-Davidson Sportster is a line of motorcycles produced continuously since 1957 by Harley-Davidson. Sportster models are designated in Harley-Davidson's product code by beginning with "XL". In 1952, the predecessors to the Sportster, the Model K Sport and Sport Solo motorcycles, were introduced. These models K, KK, KH, and KHK of 1952 to 1956 had a sidevalve ('flat head') engine, whereas the later XL Sportster models use an overhead valve engine. The first Sportster in 1957 had many of the same details of the KH including the frame, fenders, large gas tank and front suspension.

Sportster motorcycles are powered by a four-stroke, 45° V-twin engine in which both connecting rods, of the "fork and blade" or "knife & fork" design, share a common crank pin. The original Sportster engine was the Ironhead engine, which was replaced with the Evolution engine in 1986. Sportster engines, the 45 cubic inch R, D, G & W Models 1929 side-valve motors, and the 'Big Twin' side-valve motors, which were: the flathead 74.0 cu in (1,213 cc) Models V, VL etc. (1930–1936), Models U and UL (1937–1948), and the 80.0 cu in (1,311 cc) models VH and VLH (1935–1936), models UH and ULH (1937–1941) have four separate cams, sporting one lobe per cam.

The cam followers used in Sportster engines, K models, big twin side valve models, and the side-valve W model series were a slightly shorter version of the followers used in the larger motors, but with the same 0.731-inch (18.6 mm) diameter body and 0.855-inch (21.7 mm) diameter roller follower since 1929. The company used similar cam followers for decades with minor changes, from 1929 to the 1980s.

Sportster engines retained the K/KH design crankcase design, in which the transmission is contained in the same casting as the engine, and driven by the engine with a triple-row #35 chain primary drive and a multi-plate cable-operated clutch. Models since 1991 have five speeds; 1990 and earlier models had four speeds.

The engine was mounted directly to the frame from 1957 through the 2003 model year. While this system allows the bike to be somewhat lighter with more precise handling, it also transmits engine vibration directly to the rider. In 2003 Harley Davidson produced a limited number of 100 year anniversary model sportsters. They are identified by the 100-year anniversary paint schemes and plaques attached to the tinware, speedo housing and also on the engine. Sportsters released in 2004 and later use rubber isolation mounts and tie links to limit engine movement to a single plane, which greatly reduces vibration felt by the rider. Buell motorcycles built with variants of the Sportster engine have used a rubber mount system since 1987.

The Model K, from which the Sportster evolved, was the first civilian motorcycle produced by Harley-Davidson with hydraulic shock absorbers on both wheels. Common usage calls this a K Model.