It wasn’t long after the 1928 introduction of the flathead V-twins that Harley-Davidson began working on a new overhead-valve (OHV) version. This was surprising for two reasons; with the previous F-head design on the market for over 20 years, Harley was viewed as somewhat stagnant in their innovations.
The second reason was that the new engineering was taking place during the darkest part of the Great 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.
Still, Harley wasn’t exactly breaking any new ground with the valve layout: the company had produced OHV singles on and off for several years. The main innovation in the new design was a recirculating oiling system, which meant that the valve mechanisms were now enclosed- although early models were far from oil-tight.
The official name for the 1936 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. The production bikes powered by the new V-twin were designated the E-Series.
The E Solo 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 that yielded an output of about 40 hp, and was called the Special Sport Solo.
The E series ushered in the transition from the long-serving but underpowered side-valve flathead V-twin engine to the higher tech and higher powered overhead valve pushrod engines. Comparatively, Harley’s late F-head and early flathead 74s put out about 30 horsepower, and the later high-compression flatheads were rated at 36 horsepower.
The E-Series introduced two more innovations: a four-speed transmission and the now-famous tank-mounted instrument panel. Other features introduced with the 61 E series bikes tended to be overshadowed by the hype over its engine configuration and power; but they included a double-cradle frame, moly-steel springer fork, an upgraded clutch and memorable art-deco tank badges and styling. Unfortunately, despite these changes the original Knucklehead also continued an embarrassing Harley-Davidson tradition.
Oil flow had been a major issue for every engine designer in the first decades of internal combustion. In Harley-Davidson’s early F-head engines, lubrication was controlled by a system called “total loss”; the rider filled a reservoir next to the gas tank and regulated the oil flow by counting drops fall through a sight tube. Normal riding conditions called for three drops every five seconds. The oil eventually worked its way down through the engine, finally dripping onto the drive belt pulley shaft and then on the front chain ring.
Harley advertised that this system “always provided clean oil to the engine and running gear”, conveniently avoiding mention of the oil spraying onto the rider’s leg. A pump was added in 1911 to give a few extra shots to the engine under heavier strain, but since the tendency was “when in doubt, better too much than too little” that excess oil forced its way past piston rings and clogged valves, fouled the plugs, and of course lubricated the rider’s leg even more.
The later model flathead engines had introduced an oil pump linked to the throttle, but it was still tricky to regulate. The Knucklehead engine featured a dry-sump, recirculating oil system which was a valiant attempt, but it was far from perfect: three versions of oil tanks and lines were developed as modifications in 1936.
The issue wasn’t really “fixed” until 1941, when H-D developed a gear-driven centrifugal-valve pump in the recirculating oil system. This pump used a valve that provided maximum lubrication to the engine at high speed but directed the oil back to the supply tank at lower speeds.
The new Knuckleheads had other influences to contend with as well. President Roosevelt’s National Recovery Administration “strongly recommended” that employers hire additional workers rather than incurring overtime among employees already on staff, and prominent companies like Harley-Davidson couldn’t ignore that. They simply didn’t have enough time or payroll to solve the oil problem at the time.
The OHV Knucklehead engines provided forty horsepower at 4,800 rpm. The design continued the practice of shortening the stroke in relation to the bore, allowing a steady power increase from the older F-head engines. That performance improvement was necessary to take the technical lead from Indian, a company that was somewhat beaten and bruised by the economic environment of 1936 but was still Harley’s major competitor.
The bore vs. stroke issue is important to all engines; longer stroke provides more power at slower speeds in the form of torque. Shorter stroke engines achieve their power at higher speeds, and a shorter stroke gets a bigger cylinder diameter in the trade off which allows room for larger valves to get more fuel in and more exhaust out.
The Knucklehead also uses a split flywheel as a crankshaft, designed to bring the rear piston up to the top of the cylinder an instant ahead of the front. Ignition of these two cylinders is provided by a single coil with a twin lead to both plugs. As a result, both plugs fire together. This “phantom firing” was adopted simply as a matter of simplicity, and provided Harley-Davidson with its legendary vibration. Fortunately though, it is also largely responsible for the famous exhaust sound the engines produce.
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.
Despite their technical drawbacks, by 1937 the Knucklehead models began achieving increasing status as performance machines. That status was urged on by some record-setting performances. That year factory backed rider Joe Petrali took a partially streamlined twin carb EL down the beach at Daytona through the measured mile for a two-way combined average of 136.183 mph, setting a world record. 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 that same year, setting a new endurance record.
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. But with the outbreak of WWII civilian production was limited along with manufacturing materials for anything but the war effort. 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 the following year saw the dawn of the Panhead engine, and the Knucklehead was discontinued.
