Harley-Davidson Knucklehead V-Twin Motorcycles - HISTORY OF THE BIG TWIN

The H-D Knucklehead V-twin was introduced in 1936. Here’s a look at the technical evolution of the 61-cu.-in. engine and chassis of Harley-Davidson’s first OHV Big Twin.

Harley-Davidson studio side view
In 1936, Harley-Davidson's Knucklehead was claimed to produce around 36 peak hp.Courtesy of Harley-Davidson

1931 Harley-Davidson board authorizes development of a 61-cu.-in. pushrods-and-rockers overhead valve (OHV) engine. This "Knucklehead" engine, so-called because of a fancied resemblance of its rocker-box ends to the knuckles of a fist, was created to put an end to the chronic problems of the flat-head, or side-valve engines that had dominated the previous era. Harley engineers had previous experience with OHV engines dating back to the eight-valve racer, the Peashooter single, and other special projects, but this was to be The Motor Company's first production OHV.

The enduring problem of the side-valve engine is that its very hot-running exhaust valve seat and exhaust port must be cast as part of the cylinder casting itself. The more power such engines made, the hotter these parts became, distorting the cylinder out of round and making oil control and combustion gas sealing difficult. This made it attractive to do as aircraft engine and race car manufacturers had done—move the two valves into the cylinder head where exhaust heat had much less effect on cylinder roundness. There were practical arguments against OHV, notably that it was not easy to properly lubricate valves and rocker-arms without tackling the problems of circulating oil up to the cylinder head without significant leakage.


Another major advance of the new engine was its pumped, recirculating oil system. Previous engines had been lubricated by so-called “total-loss,” meaning that the rider or a drip-feed added oil to the crankcase as needed, which was flung onto moving parts by the motion of the crankshaft, and eventually lost past the piston rings and valve stems. This system had been satisfactory through an era of low to moderate power, but as riders demanded more performance, the few ounces of oil present in the crankcase became extremely hot, losing viscosity and lubricating properties. Therefore the “E” was given a modern “dry sump” oiling system, in which the pressure side of the double gear-type oil pump (located just behind the timing case at the rear of the crankcase, and driven by cross-axis gears from the crank) drew oil from a tank and circulated it to moving parts, while the scavenge section of the pump recovered flung-off oil from the crankcase and returned it to the oil tank. The presence of a circulating oil system also worked to level temperature extremes in the engine—cooling the hottest parts.

This engine was given the 90-degree included valve typical of 1930s practice, set into a chamber that was a true hemisphere. This made sense with the 6.0:1 compression ratio typical of that time, for with a long-ish stroke this could be achieved with a flat-topped piston. This left the combustion chamber completely open, allowing intake velocity to persist as flame-speeding turbulence. Valve timing is given as intake 10/51, exhaust 40/12, with .346-inch lift. A single four-lobe camshaft was gear-driven from the crank and roller tappets were carried in two bolted-on tappet blocks.

Harley-Davidson vintage
Notable issues with the Knucklehead engines built included oil leakage, valve spring breakage, poor rocker lubrication, and leakage of oil from the top end.Courtesy of Harley-Davidson

Although this engine’s heads and cylinders were of cast iron, a material of limited ability to transmit heat, the heads were carefully designed with fore-and-aft fins cooled by air that was free to pass under the valve rocker enclosures, and by several roughly circular fins below that. Iron had long been an attractive material for heads and cylinders because foundries could cast it into intricate shapes, it was hard enough for valves to seat directly against it, and it provided an oil-retaining and wear-resistant cylinder surface. Crankcases were cast aluminum.

The “E” engine, with its improved ability to handle heat, its circulating oil system, and its more direct intake and exhaust flow paths, essentially doubled the power available to the rider. The E-model listed at 37 horsepower, the EL at 40 hp, both at 4800 rpm.

1936 Spring announcement of 61E at bore and stroke of 3 5/16 x 3 1/2 = 60.3 cu. in. Cradle twin down-tube frame, heavier fork, horsepower is around 36 on 6.0:1 compression. A four-speed constant-mesh transmission (Indians were still shifted by crashing gear teeth into mesh sideways), multi-plate clutch, 565 pounds wet with one gallon of oil. Dry sump pumped lube system. A single four-lobe cam is used, requiring non-parallel unequal-length pushrods. Much of the detail design work came from Hank Syvertson and Joe Petrali. It was released over their objections in June, despite fairly severe oil leakage, chiefly from the rocker boxes. Much of the reason for this chronic leakage was the complex shape of the surfaces to be sealed, which was over-ambitious. When heads heat up and cool down, gaskets are subjected to expansion and contraction of the parts they seal, and this remains a difficult problem to this day. Rocker boxes were separate, bolted to the finned heads via four cast-on lugs (Andrew Wilgoos at Pratt &Whitney had patented cast-integral rocker boxes). The iron head was retained by five short fasteners, and had five horizontal fins. The front exhaust port turns 100-deg to face downward. Linkert float carburetor, used until 1965. Cylinders were base-bolted to the crankcase by four short bolts each.

Heads are copiously finned on top with 13 fore-and-aft fins, which was at the time best practice. Access of cooling air to these fins was enabled by providing room under the rocker enclosures.

The crankshaft continued existing practice in having all-rolling bearings. Each cast-iron flywheel was supported by its own stub mainshaft, retained by taper, Woodruff key, and nut, and the two flywheels were joined by an alloy steel crankpin retained in the same manner. The crankpin was 1.125-inch diameter with a 1.625-inch race for 1/4-inch rollers. Flywheel outer diameter was 8.125 inches through 1940. Fork-and-blade connecting rods are used as in previous models, placing the cylinder axes in a common plane. The alternative, as used on V-8 auto engines, is to place the connecting rods side-by-side on a common crankpin, but this creates a side-to-side rocking couple because of the necessary cylinder offset.

The first engines built had problems with oil leakage, valve spring breakage, poor rocker lubrication, and leakage of oil from the top end. Spring metallurgy was improved, rockers redesigned, and repair kits were sent out to dealers to fix the approximately 1900 affected new bikes.

1938 At first, valve springs and the projecting valve stems were unenclosed to assure good cooling, as was common in OHV engines of other makers at the time. In this year full enclosure was adopted.

1939 Stiffer valve springs are adopted, and splined oil pump drive gears.

1940 Knucklehead flywheel diameter increases to 8.500 inches, remains 8.5 through 1999 Harley-Davidson Evolution V-twin.

1941 61 cu.-in. increased to 74 cu.-in. Knucklehead, is called FL. Cylinder bore dimensions are 3.4375 x 3.96875" = 73.7-cu. in. In carpenter's measure, this was 3 5/16 x 3 31/32 inches.

1945 The Postwar period—It is said that while Britain exported 70 percent of its motorcycle production, Harley-Davidson was unable to fill orders because of still-in-force quotas on steel, aluminum, and rubber. Reduced tariffs were a part of postwar Marshall Plan Aid and this, it was argued, made British motorcycles artificially cheap (might also have something to do with the British pound being $5 before the war and $3 after). It is also argued that aluminum, which had seemed expensive before the war, became cheap as soon as thousands of wartime aircraft and engines were scrapped. Wartime aluminum price had been fixed at 15 cents a pound.

Changes were coming that would soon compel redesign of the “Knuck.” Once World War II ended in August, 1945, the US economy turned to civilian matters—rural dirt roads were black-topped and more four-lane highways like the Pennsylvania Turnpike were built, allowing motorcyclists to ride faster and farther. The more power an engine makes, the greater the heat it must dissipate.