In fact, the manufacturer had an entire line of head-themed engines, including the Knucklehead, Panhead, and Shovelhead, with some featuring in the rarest Harley-Davidson motorcycles ever built.

The Harley-Davidson EL motorcycle was nicknamed the Knucklehead ... another entry into its "head" series of engines, the Shovelhead, given its name because of its shovel-shaped rocker covers.