Which Harley-Davidson engine was introduced in 1936?

The Harley-Davidson Knucklehead engine was introduced in 1936.

The Knucklehead was Harley-Davidson’s first production overhead-valve V-twin engine. Its name came from the distinctive shape of the rocker-box covers, which resembled a clenched fist or knuckles. The engine replaced the company’s older flathead and side-valve designs in the high-performance portion of Harley-Davidson’s lineup.

The Knucklehead is also identified by engine-displacement designations, including 61 and 74 cubic inches. It helped establish the overhead-valve architecture that later Harley-Davidson engines developed further. The name “Knucklehead” is a later popular nickname, rather than the formal name used in every original company document.

Collectors sometimes confuse it with the Panhead, which arrived after World War II in 1948, or the Shovelhead, introduced in 1966. The sequence is useful: Knucklehead, Panhead, and Shovelhead describe successive generations of Harley-Davidson big-twin engines.