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.

Prior to this breakthrough, Harley-Davidson only used a side-valve flathead engine. However, with the new overhead valves, the Knucklehead brought unprecedented power, offering 40 hp with 61 cubic ...

Harley-Davidson has had a rough patch these past ... and replacing it with what came to be known as the Knucklehead. It stayed in production for a little over a decade, right through the war ...

and the "Knucklehead" engine in 1936, said Bill Jackson, archivist at the Harley-Davidson Museum. Need a break? Play the USA TODAY Daily Crossword Puzzle. “People inside the company are ...