It is perhaps one of the most coveted types of American Classic motorcycles in existence.The 61 cubic-inch E, ES and EL Harley-Davidson models ushered in the transition from the long-serving but under-powered side-valve flathead V-twin engine to the higher tech and higher powered overhead valve pushrod engines.
While the new 61 E series model included a number of other product innovations when it was introduced in 1936, it was its distinctive engine that gave it an unmistakable appearance and unbeatable nickname: the “knucklehead.”
Harley-Davidson Knucklehead – ‘Clinched Fist’
The polished rocker covers to many resembled huge clenched fists to give the motor its name. Indeed, today many enthusiasts can instantly identify a knucklehead when they see one, but fewer recall the model designations that applied.In the context of the turbulent times it was born into, the introduction of any new model motorcycle had to seem like small potatoes. The world was still in the swoon of a great economic depression, fascism gripped much of Europe, war clouds were gathering in both Europe and Asia, and FDR was doing his best to keep the country from starving.But, as time went on and things got better, the knucklehead models attained ever-increasing status as performance machines. That status was urged on by some record-setting performances.For example, in 1937, factory backed rider Joe Petrali took a partially streamlined twin carburetor EL down the beach at Daytona through the measured mile for a two-way combined average of 136.183 mph, setting a world record!Later in the year, in an effort to showcase the engine’s reliability, as well as speed, an EL covered 1,825 miles in 24 hours at an average speed of 76 mph, setting a new endurance record.The E Solo model in the series was the base model with an output of about 37 hp using a compression ratio of 6.5:1; the ES model was the same, targeted for use in sidecar rigs, the EL came with a 7.0:1 compression ratio and that higher compression yielded an output of about 40 hp and was called the Special Sport Solo.Other features that were introduced with the 61 E series bikes that tended to be overshadowed by the hoopla about its engine configuration and power were things like a pressurized dry sump lubrication system that boosted oil pressure as engine speed increased, a double-cradle frame, moly-steel springer fork, an upgraded clutch and four speed transmission, tank mounted dashboard that included mechanical ammeter and oil pressure gauge and memorable art-deco tank badges and styling.In 1936 1,704 knuckleheads were produced, 1,526 of which were the hotter EL version, while only 152 of the E and but 26 of the ES versions were built. Imagine making a barn find of one of those precious few 1936 ES versions today!By 1938, the Solo E model was dropped from the line and the remaining bikes got warning lights to replace the oil pressure and ammeter gauges, a completely enclosed rocker box, better brakes and transmission.In 1941, the knucklehead design was expanded to include the 74 ci (1,200 cc) bikes and other mechanical improvements were made across the product line. With the outbreak of WWII, civilian production was limited and manufacturing materials for anything but the war effort was, as well.How far did production fall during the war? In 1940, EL and ES production totaled 4,069 units—in 1943, it fell to only 158. By 1947, production had rebounded to 4,354 EL and ES bikes, but 1948 saw the dawn of the Panhead engine and the sunset of the Knucklehead.