By 1936, overhead valves had become the default configuration for British and European motorcycles, while American manufacturers Harley-Davidson and Indian still produced side-valve or flathead engines. This wasn't necessarily a bad thing, as side-valve engines were generally simpler in that they had fewer moving parts, making them cheaper to produce, easier to maintain, and much more reliable, although they were less powerful and tended to run hot, which posed other problems. Although the flathead engine continued until 1973, in the Servi-Car, in 1936, the future looked bright for Harley-Davidson with the introduction of the Knucklehead engine, which featured overhead valves and pushrods mounted inside two external tubes that ran down the side of each cylinder. The Knucklehead engine is the origin of the engine configuration that still exists today in the modern Harley-Davidson Milwaukee 8 engine.

Origins of Knucklehead

By the early 1930s, the limitations of flathead, or side-valve, cylinder head design were becoming increasingly apparent, especially as efforts were made to extract more power from engines. The problem lay in the fact that the valves were integrated into the main cylinder block, where they were exposed to excessive heat buildup. This was partly due to the use of cast iron for casting, a material that does not dissipate heat well. This excessive heat distorted the cylinder, including the valve train, resulting in poor combustion sealing and ineffective oil control. Moving the valves to the cylinder head removed a major heat source—exhaust gases—from the cylinders. However, overhead valves were not a perfect solution, as lubricating the valves and swingarms required rethinking oil circulation without causing excessive leakage.

Continuous Development and Eventual Disappearance

To solve the lubrication problem, Harley-Davidson developed an oil pumping and recirculation system, with a remote reservoir that held the oil. Previously, engines were lubricated by a total-loss system, in which oil was dripped into the crankcase, thrown by the crankshaft onto the moving parts, and lost by burning in the cylinder or escaping through the valve stems. With low-powered, lightly used engines, this system was just about adequate, but as power increased, there was not enough oil in the crankcase, which became excessively hot and lost much of its lubricating properties. The Harley-Davidson Model E , on which the Knucklehead engine first appeared, had a dry-sump lubrication system, with oil circulated to the moving parts and the excess returned to the oil reservoir. This allowed, among other things, to reduce the engine temperature.

Post-War and End of the Knucklehead

Although cast iron has poor heat dissipation qualities, as noted above, this problem was largely overcome by careful attention to the design of the cooling fins on the cylinder head and cylinders. Cast iron was not without its drawbacks: foundries had extensive experience with the material and could form it into complex shapes. Furthermore, it was hard enough for valves to seat directly in it, without the need for valve seats, and to form a wear-resistant inner cylinder surface. While cast iron added weight, this was not a major problem, as the new knucklehead engine produced about twice the power of the flathead.

Introduction and Problems of 1936

The introduction of the Model 61E with the Knucklehead engine represented a major step forward for Harley-Davidson. The increase in power was the most obvious benefit, but the gearbox was now a constant-gear unit, much smoother than the Indian's "crash" gearbox. The fork-and-blade connecting rods, which placed the axes of the two cylinders on the same plane, were retained. However, as with many new models, the Model E was launched before the engine was fully tested and ready. Oil leaks were particularly problematic, mainly from the rocker boxes, because the excessively complex shape of the sealing surfaces caused the seals to fail after numerous heating and cooling cycles. Initially, the valve springs and stems were exposed, but they were completely enclosed starting with the 1938 model year.

Continuous Development and Eventual Disappearance

World War II began in 1939, and direct U.S. involvement began in late 1941, allowing Harley-Davidson engineers to continue developing the Knucklehead engine until then. In 1941, the Knucklehead's displacement was increased to 74 cubic inches, up from the original 61 cubic inches, providing even more power and flexibility.

Immediately after the war, Harley-Davidson was unable to fulfill orders due to the quotas still in place on raw materials—steel, aluminum, and rubber. However, shortly afterward, aluminum became freely available due to the large amount of discarded wartime mechanical equipment, which ironically marked the beginning of the end for the Knucklehead engine.

With the end of the war, the American economy was able to focus again on domestic and civilian affairs, particularly the highway system, with many paved roads and the construction of interstates. These improvements significantly increased average speeds, which exposed the weaknesses of many pre-war engines. More speed required more power, and more power generated more heat, which was problematic for the all-iron Knucklehead engine, which could not handle this excess heat.

In response to these needs, Harley-Davidson's next engine featured iron cylinders but aluminum heads, with a rocker cover resembling an upside-down saucepan. Thus, the Panhead engine was born, marking the end of the Knucklehead era.