The Flathead was named after the flat vented tops of the cylinder heads– 45 cubic inches of displacement producing about 30 to 35 horsepower. The engine proves to be so reliable that variations are available on Harley-Davidons as late as 1973.
On the eve of WWII, Harley-Davidson introduced the 61 OHV V-twin with “knuckly ” looking rocker boxes. The 1936 through 1940 models displaced 61 cubic inches. In 1941 a 74 cubic inch displacement version was added. The knucklehead generated 40 horsepower in the 61 cid and about 45 hp in the 74 cid models, offering noticeable extra punch.
The Panhead was named after the “pan-like” covers. It ran a quieter and cleaner than it’s predecessor the Knucklehead. New features are eventually added– aluminum heads (for lighter weight and more power) and hydraulic valve lifters that supplied oil through rocker arms for better cooling and engine life.
By 1966 the Panhead underwent still more improvements, the most noticeable being the distinctive sleek, slightly curved top– the Shovelhead. At first glance this looks unmistakably like a coal shovel with the pushrods resembling the handle, and the rocker box looks like the shovel head itself. The design provided more than a unique look– it’s slightly more powerful than the panhead engine which it replaced. However, there was some debate over the weight of the new Shovelhead– some still steer away from it because they feel it is slightly slower than some other Harley-Davidson engines.