History is definitely on the side of the mighty Sportster. It's the lightest, sleekest, and inch-for-inch the powerhouse of the extensive Harley-Davidson history. Unveiled in 1957, the unit construction mill with trapdoor gearbox was the evolution of the K-models that ripped up the street and strip. A major innovation of the XL motor was the inclusion of overhead valves and the addition of the iconoclastic Shovelhead-style rocker-arm covers. Retained from the K were the four-cam configuration and transmission cavity cast into the crankcase. The quad-cam arrangement meant better valve control through reduced pushrod angles, and the one-piece motor/tranny case made the combo lighter and more rigid than a Big Twin.