Celebrate the 80th anniversary of the engine that changed the motorcycle world.Motorcycle technology lagged far behind automotive technology since the earliest days of the internal-combustion engine. All that changed in 1936 when Harley-Davidson introduced the Model EL. For the first time ever, a company was manufacturing a high-performance overhead-valve engine for the masses.And what an engine it was -- in addition to bringing state-of-the-art technology to the motorcycling world -- a work of art. Because of the shape of its rocker covers, the engine was given a nickname to match its looks: the Knucklehead. The technology used in this engine was so advanced that it laid the foundation for every future Harley-Davidson motorcycle, including the current models built in the 21st century. To this day every cruiser style motorcycle still adheres to the shape of that original Knucklehead.Harley-Davidson Knucklehead: Eighty Years tells the entire Knucklehead story, from the very first overhead-valve V-twin Harley produced for the public through the post-war models, continuing right up until today, when aftermarket manufacturers reproduce complete Knucklehead crate engines because of its continuing popularity.