Introduced in 1936, Harley-Davidson's "Knucklehead" was the company's first production overhead-valve engine; it's the great-grandfather of today's Evolution engine.The engine was just one of many innovations associated with the 1936-47 era, all of which are covered in Harley-Davidson Knuckleheads. Illustrated with beautiful new-photography and archival photos from the Knucklehead era.