In 1936, Harley-Davidson debuted the 40 bhp, 61 cubic inch EL “hemi” overhead valve model. Thanks to the prominent bumps on the cylinder heads, it quickly earned the nickname “Knucklehead”, and it was a name that stuck. First planned as early as 1931, the overhead valve engine and its development had been delayed since 1934 by the economic climate of the Great Depression, but after its initial teething problems wore mostly sorted out, it remained a solid performer in Harley’s lineup, with a 50 bhp 74 cubic inch model added 1941. A four-speed gearbox was offered from the start, and 61 ci and 74 ci knuckleheads were built until 1947.

While Harley-Davidson had designed and built overhead-valve race engines in the past (some even with four valves and double overhead camshafts), this was the company’s first fully overhead valve street engine. The base model was dubbed the “E”, while “EL” became the name of the Special Sport, and “ES” was given to bikes set up for use with a sidecar. In addition to being the first Harley with overhead valves, it was also the company’s first bike to come with a speedometer as standard equipment.

As for the initial teething problems with the new engine, these were mainly characterized by trouble regulating oil flow to the valves. By 1937, however, a re-circulating oil system had alleviated the issue, and by 1941 the oil supply problem was solved with a gear-driven centrifugal pump with a bypass valve for higher up in the rev range.

The 74 ci model that was introduced in 1941 was supposedly for sidecar use, but it was more likely a step to face challenge of the big Indian Chiefs, which were notably fast. This was the model that was dubbed the FL. The year 1941 also saw the clutch grow in size by 65 percent as well as an improved air cleaner, quieter muffler and 50 percent larger rear brake. Gauges also became black-faced and both the hand-shift and foot-clutch mechanisms were improved.

16-inch tires were introduced in 1940, and new cylinder heads had bigger ports. The following year’s 74 cubic inch FL then required larger crankcases and flywheels. The US entry into the Second World War shortly thereafter meant that American manufacturing attention turned to military production, and only a few OHV models were produced during hostilities.

When civilian production resumed in 1946, sales of the E and F series boomed (still the same base, sports and sidecar models). In all, over 11,000 Knuckleheads were sold in 1947, which made up over half of Harley-Davidson’s total sales of 20,000. In 1936, the number was closer to 20 percent. That sales total of 1947 was the company’s best since before the Great Depression, but the company didn’t content itself with coasting on its strong postwar sales. For 1948, Harley had an ace up its sleeve, the “Panhead”.