It wasn't long after the introduction of the flathead V-twins that Harley-Davidson began working on a new overhead-valve version. This is surprising not only because so little time had elapsed since the flathead's debut (the previous IOE design was on the market for over 20 years), but also because the initial decision and engineering work took place during the darkest days of the depression.
Of course, Harley wasn't really breaking any new ground here, as the company had produced overhead-valve singles on and off for many years. But along with the new motor's overhead valves came a recirculating oiling system, and that was new to Harley-Davidson. Because of it, the overhead-valve mechanisms were now enclosed-though early models proved far from oil-tight.
Harley's contemporary flathead Big Twins displaced 74 and 80 cubic inches (the latter added in late 1935), but the new motor was sized at only 61 cubic inches.
Due to its more efficient overhead-valve design, however, it put out more power. According to factory engineering figures, late F-head and early flathead 74s both put out about 30 horsepower. Later high-compression flatheads were rated at 36 horsepower, but that was still shy of the 40 horsepower claimed for the new overhead-valve motor.
The official name for the overhead-valve V-twin was the "61 OHV," but riders soon dubbed it the "Knucklehead" due to its valve covers, which looked like fists with two knuckles sticking out. Motorcycles powered by the new V-twin were designated the E-Series: E models had lower 6.5:1 compression giving 37 horsepower, while ELs had 7:1 compression and 40 horsepower.
Aside from the OHV motor, the E-Series introduced two more innovations: a four-speed transmission and the now-famous tank-mounted instrument panel. Flathead Big Twins offered the four-speed as an option in 1936, and all V-twins (including the 45) adopted the tank-mounted instrument panel in 1937, along with the OHV's recirculating oiling system. With that, designations changed: The flathead Big Twins were now the U-Series rather than the V-Series, and Forty-fives were the W-Series instead of the R-Series.
In 1941, a larger 74-cubic-inch version of the OHV appeared under the F-Series designation. Shortly thereafter the 80-inch flathead was dropped, but the 74 flathead remained available through 1948.
World War II prompted both a military version of the Forty-five and a special horizontally opposed flathead twin with shaft drive that was designed for desert use. The former was called the WLA, and 88,000 were built for use by U.S. troops. The latter XA model didn't fare as well; only 1,000 were built, and none saw action overseas.
As it turns out, the revered Knucklehead lasted only a dozen years on the market (and World War II took a chunk out of that), but its influence was far greater than the figure would imply. It formed the basis for all Big Twins produced since, and that's a legacy that can't be ignored -- or forgotten.
Continue reading for more Harley-Davidson history.