The knucklehead is a retronym used by enthusiasts to refer to a Harley-Davidson motorcycle engine, so named because of the distinct shape of the rocker boxes. The engine is a two-cylinder, 45 degree,
Harley-Davidson Knucklehead engine at the Harley-Davidson Museum
Harley-Davidson knucklehead motor
The Panhead is an overhead-valve Harley-Davidson motorcycle engine, so nicknamed because the rocker covers resembled cooking pans. The engine is a two-cylinder, two-valve-per-cylinder, pushrod V-twin,
Harley-Davidson Knucklehead engine
…replacing the Flathead-engined VL model in 1936 as HD's top-of-the-line model. The engine was manufactured until 1947 and was replaced by the Panhead engine in 1948. The Knucklehead-engined models were originally referred to as "OHVs" by enthusiasts of the time and in Harley's official literature…
Harley-Davidson Panhead engine at the Harley-Davidson Museum
Harley-Davidson panhead motor
Replicas of the Captain America bike and Billy Bike at the Harley-Davidson Museum in Milwaukee