Harley-Davidson’s 1936 overhead-valve V-twin engine was nicknamed the Knucklehead.
The nickname refers to the shape of the rocker-box covers, whose prominent forms were said to resemble a clenched hand’s knuckles. Officially, the engine was an overhead-valve V-twin, but the informal name became far more memorable among riders, historians, and collectors.
The Knucklehead represented an important technical step for Harley-Davidson because overhead valves improved breathing compared with the company’s earlier flathead designs. It powered motorcycles in 61- and 74-cubic-inch forms and remained in production through the 1947 model year.
Knucklehead, Panhead, and Shovelhead are sequential nickname traditions, not interchangeable names. The Panhead followed in 1948, while the Shovelhead arrived later in 1966. Their names likewise came from the appearance of their rocker covers.