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. another cool and unique feature on the new Harley Knucklehead was push rod tubes running from the crankcase up to the top of the cylinder heads. The cylinder heads were covered by allot covers that also covered the rockers. 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. This method of engine lubrication is known as "dry sump lubrication."

Why was the new motor called a "Knucklehead?" Clench your fist, look at the back of your hand, and you'll see why this was called the Knucklehead. The polished nuts that retain the rocker arms are the knuckles and the tendons that run down the back of your hand are the push rod tubes. With the overhead valve gear and recirculating lubrication system, the Knucklehead was a giant leap forward for Harley-Davidson.

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 1936 61E Harley-Davidson was the first Harley to use the new Knucklehead engine. The Harley 61E used a medium compression Knucklehead while the 1936 61EL Harley use a high compression Knucklehead producing more power. Many Harley-Davidson riders feel the Knucklehead was the motor that put the Indian Motorcycle Company out of business.

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.

The Harley Knucklehead is very popular with chopper enthusiasts. The engine design and style look great in a chopper and Harley Davidson's reputation for building cool, tough and powerful motorcycles and engines make the Knucklehead the motor of choice for custom choppers. The Knucklehead has a raw look of power and produces 40 to 45 horsepower in stock trim.