A knucklehead is an engine on a Harley-Davidson motorcycle that has a valve cover that resembles a fist with two knuckles sticking out. The term was created by consumers; the company referred to the engine as the 61 OHV.
The knucklehead engines replaced Harley-Davidson's flathead side-valve engines in 1936. They were the company's first overhead-valve V-twin engines. The overhead valves gave the motorcycles more power, with a displacement of 61 cubic inches that was upgraded to 74 inches starting in 1941. Harley-Davidson knuckleheads also had a recirculating oil system. In 1948, the company replaced the knucklehead engine with panhead motors that utilized aluminum rather than cast-iron heads.