Thanks to Panhead Jim for allowing us to share parts of this excellent post on Harley Knucklehead production data from his blog http://www.ridingvintage.com/
Note: These production numbers are based on fiscal rather than calendar year model production numbers. To date, Harley-Davidson has not released an official listing of production numbers by model year.
For the 1936 model year, Harley-Davidson released a completely redesigned motorcycle which was powered by an all new 61 ci OHV (Over Head Valve) engine. This engine was nicknamed the “Knucklehead” due to the shape of the rocker covers, which some thought resembled the knuckles of a clinched fist. It had many advancements over the earlier side valve engines, but most importantly it was the first Harley engine to use a recirculating oil system. Harley’s engineers took advantage of this consistent source of pressurized oil and built an engine with higher compression, larger valves and a shorter stroke. The result was an OHV engine that produced 40 bhp at 4,800 rpm. Compare that to the larger 80 ci side-valve engine of the same era that only produced 39 bhp. The Knucklehead Era lasted from 1936 through 1947 models.