The Harley Davidson motorcycle Knucklehead engine, so named because of the shape of its rocker covers, came with two significant new features when it was introduced in 1936.  The first was overhead valves, which boosted power from the V-twin unit; and the second was a single camshaft arrangement that would be used on Harley Davidson motorcycle big twins for the next sixty years. The Knucklehead was also the first Harley Davidson motorcycle engine to incorporate a reticulating oil system, where oil is constantly fed through the engine rather than just burned off.

A streamlined Harley Davidson motorcycle powered by the new Knucklehead engine received nationwide publicity in March 1937, by achieving a record speed of 136 mph when ridden by Joe Petrali on Daytona Beach, Florida. With the Knucklehead engine released just the year before, it was confirmation of the new engine's pedigree.

The main competition for the Harley Davidson motorcycle Knucklehead was the Crocker motorcycle. Built in California, the Crocker was produced in limited numbers and was one of the few motorcycles capable of outrunning a good Knucklehead. The Crocker was built in the classic style of an American V-twin just like the Harley Knucklehead.