Aug 02, 2023
The Knucklehead was one of the first modern motorcycle engines ever produced, and provided a foundation for Harley-Davidson to build upon
By 1936, overhead valves had become the default layout for British and European motorcycles, while the American manufacturers Harley-Davidson and Indian were still producing side valve, or flathead, engines. This wasn’t necessarily a bad thing as side valve engines were generally simpler in that there were less moving parts which made them cheaper to produce, easier to maintain and much more reliable, even though they were less powerful and tended to run hot, bringing alternative issues. However, even though the flathead engine lasted to 1973, in the Servi-Car, in 1936 the future beckoned at Harley-Davidson when the Knucklehead engine appeared, with overhead valves and push rods mounted inside twin external tubes running up the side of each cylinder. The Knucklehead engine pioneered the engine layout that exists to this day in the modern Harley-Davidson Milwaukee 8 engine.
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By the early 1930s, the limitations of the flathead, or side valve, cylinder head design were becoming increasingly apparent, especially as more and more power was coaxed out of the engine. The problem lay in the valves running in the main cylinder casting, where they were subject to excessive heat build up, which was partly due to the use of cast iron for the casting, which isn’t a great heat dissipater. The excess heat would distort the cylinder, including the valve gear, leading to poor combustion sealing and oil control. Moving the valves to the cylinder head removed a large source of heat - the exhaust gases - from the cylinders. Overhead valves weren’t the be-all and end-all, as lubricating the valves and rocker arms required re-thinking oil circulation without excessive leakage.
In overcoming the lubrication problem, Harley-Davidson developed a pumped, recirculating oil system, with a remote reservoir holding the oil. Previously, engines had been lubricated by a total-loss system, whereby the oil was drip fed to the crankcase, flung about by the crankshaft onto moving parts and lost by being burnt in the cylinder or leaking through the valve stems. With low-power and under-stressed engines, that system was just about good enough but, as power outputs rose, there wasn’t sufficient oil in the crankcase, leading it to get far too hot and lose much of its lubrication properties. The Model E Harley-Davidson, in which the Knucklehead engine first appeared, had dry sump lubrication, with oil being circulated to moving parts and the excess being returned to the oil reservoir. Among other benefits, this reduced the temperature in the engine.
Even though cast iron has poor heat dissipation qualities, as mentioned above, this problem was largely overcome with careful attention to the design of the cooling fins on the cylinder head and barrels. Cast iron wasn’t all bad: foundries had a lot of experience with the material and could cast it into intricate shapes. Also, it was hard enough for the valves to seat directly onto it, without the need for valve seats to be inserted, and it was hard enough to form a wear-resistant interior cylinder surface. If cast iron added weight, then the wasn’t a huge problem, as the new Knucklehead engine developed roughly twice the power of the flat head.
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The Model 61E with the Knucklehead engine was a huge step forward for Harley-Davidson. Increased power was the most obvious benefit, but the gearbox was now a constant-mesh unit, which was infinitely smoother than the Indian’s crash gearbox. Fork-and-blade connecting rods, which place both cylinder axes on the same plane, were retained. However, in line with many new model introductions, the Model E was launched before the engine had been fully tested and was ready. Oil leakage in particular was a problem, mainly from the rocker boxes, caused by overly-complex shape of surfaces to be sealed, leading to gasket failure after many cycles of heating and cooling. At first, valve springs and valve stems were exposed, but these changed to fully-enclosed for the 1938 model year.
The Second World War started in 1939 and direct U.S. involvement commenced in late 1941, which gave Harley-Davidson engineers time to continue development of the Knucklehead engine up to that point. In 1941itself, displacement of the Knucklehead was increased to 74 cubic inches, from the original 61 cubic inches, giving even more power and flexibility. Immediately post-War, Harley-Davidson was unable to fill orders due to still in-force quotas on raw materials - steel, aluminum and rubber. However, before long, aluminum became freely available, due to the large numbers of wartime mechanical equipment being scrapped and this, ironically, spelled the beginning of the end for the Knucklehead.
With the end of the war, the U.S. economy could turn once again to domestic and civilian matters and one area of the country that received attention was the road system, with many roads being properly tarred and highways built. With these improvements, average speeds went through the roof and exposed the weaknesses of many pre-war engine designs. More speed needs more power and more power creates more heat, which is something the all-iron Knucklehead engine didn’t need and couldn’t cope with. Thus, the next Harley-Davidson engine had iron barrels but aluminum cylinder heads, complete with a rocker cover that resembled an up-turned cooking pan. The Panhead engine was born, and the Knucklehead came to the end of its life.
Harry has been writing and talking about motorcycles for 15 years, although he's been riding them for 45 years! After a long career in music, he turned his hand to writing and television work, concentrating on his passion for all things petrol-powered. Harry has written for all major publications in South Africa, both print and digital and produced and presented his own TV show called, imaginatively, The Bike Show, for seven years. He held the position of editor of South Africa's largest circulation motorcycling magazine before devoting his time to freelance writing on motoring and motorcycling. Born and raised in England, he has lived in South Africa with his family since 2002. Harry has owned examples of Triumph, Norton, BSA, MV Agusta, Honda, BMW, Ducati, Harley Davidson, Kawasaki and Moto Morini motorcycles. He regrets selling all of them.