The Harley Davidson Knucklehead is a revered motorcycle engine with a rich history dating back to the mid-1930s. Named for its distinct shape characterized by bulbous rocker boxes sat on top of the cylinder head, this particular motor marked a significant milestone for the company as it surged ahead of its then-competitor, Indian.
Developed during the Great Depression, the Knucklehead was introduced with the 1936 Harley-Davidson EL, an overhead valve V-twin motorcycle. It replaced the Flathead engined VL model.
This new 45-degree V-twin incorporated pushrod-actuated inlet and exhaust valves, setting new standards in performance and design for American motors. With the debut of models like the 61 cubic-inch E, ES, and EL, the Harley Knucklehead helped solidify the brand’s reputation in the motorcycle world.
Despite the economic hardships of the era, Harley Davidson’s commitment to engineering ensured this powerful motor quickly gained a following, and today is one of the most coveted types of American classic motorbikes.
History and Development
Depression Era
In the 1930s, Harley Davidson found itself struggling against its competitor, Indian. Hank Syvertson and Joe Petrali were given the job of designing an overhead-valve v-twin engine that would get them back on a par with their main rivals.
Engineers Hank Syvertson and Joe Petrali outdid themselves and improved on the previous Flathead so much that in 1936 Harley Davidson actually pulled ahead of Indian with the introduction of the Knucklehead. This new big twin was developed in the midst of the Great Depression.
The Knucklehead, initially known as the Model 61E, featured a 61 cubic inch V-twin motor with a 6.0:1 compression ratio and 36 horsepower. It was considered highly sophisticated for its time, even by Harley Davidson’s own admission, primarily built as a developmental motorcycle.
Early models had oil leaks at the rocker cover and valve springs were prone to breaking. Repair kits were sent to the dealerships to fix any motorcycle returned with these problems.
The Knucklehead improved on its predecessor, the Flathead in several ways. The Knucklehead was an overhead valve (OHV) V-twin with two valves per cylinder, which was a significant improvement over the side-valve engines like the Flat head.
The Harley Davidson Knucklehead also had a new recirculating oil pump system that returned the lubricant from the crankcase back into the oil tank. This provided better lubrication to the valves and rocker arms in the cast-iron heads.
Additionally, the Knucklehead had a larger displacement and a higher compression ratio, which resulted in a significant increase in performance.
The Knucklehead made 37-40 HP in stock trim, which was double that of the old Flathead. Overall, the Knucklehead was a significant improvement over the Flathead and it helped to establish Harley Davidson as a leader in the motorcycle industry.
Post World War II
In 1941, Harley Davidson expanded the knucklehead design to include the 74 ci (1,200 cc) bikes, and implemented mechanical improvements across their product line. The Knucklehead continued to be manufactured until 1947, when it was replaced by the Panhead motor in 1948.
During World War II, civilian production of the Knucklehead was limited due to Harley-Davidson’s focus on providing military motors. The Knucklehead’s nickname arose from the California chopper culture of the late 1960s and has since become an iconic part of Harley Davidson’s history.
Engine and Performance
The Harley Davidson Knucklehead motor was the company’s first production overhead valve V-twin. This two-cylinder, 45-degree motor features pushrod actuated overhead valves with two valves per cylinder.
Lubrication System
The Knucklehead introduced an improved circulating oil system, which was a significant upgrade from previous models. This contributed to the engine’s ability to handle heat and led to longer life and better reliability. It also provided more efficient cooling for the motor, enhancing its overall performance.
Horsepower
While specific HP ratings for the Knucklehead can vary, the “E” motor, with its improved heat management and more direct intake and exhaust flow paths, essentially doubled the power available to the rider compared to previous models. This increase in HP allowed for a more powerful and enjoyable riding experience, making the Knucklehead a popular choice among motorcycle enthusiasts.
Model Variations
EL Model
The EL of the Harley Davidson Knucklehead was first introduced in 1936. It featured an overhead valve, 45-degree V-twin with a displacement of 61 cubic inches (988 cc). The bore and stroke for this motor measured 3.31″ x 3.50″, and it produced 40 HP at a compression ratio of 7:1.
Equipped with a 1.25-inch Linkert carburetor, the EL utilized a duplex chain for its primary drive, and a chain for its final drive. The transmission consisted of a 4-speed, hand-shift/foot-clutch mechanism, and the bike was fitted with drum brakes on both the front and rear.
FL Model
The FL of the Knucklehead series was introduced in 1941, featuring a slightly larger 74 cubic inches displacement. This increase in size allowed for improved performance and power output, making the FL a popular choice among Harley Davidson enthusiasts.
Other improvements that had been made by this time included stiffer valve springs an totally covered motor top end.
Both EL and FL models were significant precursors to the subsequent Panhead and Shovelhead engines that Harley would later release.
Design and Features
Rocker Boxes
The Harley Knucklehead is known for its distinctive shape of the rocker boxes. These boxes feature a unique “knucklehead” design, characterized by bulging, knobby contours on top of the cylinders. This feature has made the engine a popular choice among chopper builders and motorcycle enthusiasts.
Foot Clutch
Another notable design aspect of the Knucklehead is the foot clutch. This system allows riders to shift gears using their foot, providing more control and flexibility while riding. The foot clutch is a classic feature that is still appreciated by many in the motorcycle community today.
Art Deco Styling
Harley Davidson’s Knucklehead engine also showcases Art Deco-inspired styling. This design approach, popular in the 1930s and 1940s, is marked by streamlined, symmetrical shapes and bold geometric contours. This era-defining aesthetic can be seen in various aspects of the Knucklehead, from its engine design to the overall motorcycle build.
Sidecar Options
When the Harley Davidson Knucklehead was first introduced, sidecar options were available for riders who wanted to add an extra level of functionality and style to their motorcycles. These sidecars were designed to match the motorcycle’s overall Art Deco aesthetic and enhance the typical riding experience. While sidecar usage has decreased over time, it remains an intriguing aspect of the Knucklehead’s design history.
Knucklehead Successors
Panhead
The Panhead engine succeeded the Harley Davidson Knucklehead in 1948 and was produced until 1965. It got its name from the distinct shape of its rocker covers, resembling upside-down pans. The Panhead engine had several improvements, including aluminum cylinder heads, which reduced engine weight and improved cooling.
This new engine had hydraulic valve lifters, making it quieter and less demanding in terms of maintenance. The Panhead also saw the introduction of telescopic front forks, improving suspension and handling on Harley Davidsons at the time.
Shovelhead
The Harley Shovelhead succeeded the Panhead in 1966 and was produced until 1984. This engine inherited its name from the pan-like appearance of the rocker covers, which resembled a shovel head. The Shovelhead brought further improvements to the Harley motorbikes, such as increased capacity and more power.
Some of the Shovelhead’s key features were a larger bore size and better oil circulation, translating to better performance and reliability. The motor was initially available with either a generator- or alternator-style charging system, but eventually, the generator version was phased out.
Evolution
The Harley Davidson Evolution engine was the last new motor of the 20th century for Harley and improved on the Shovelhead in several ways. It had a more efficient design with aluminum heads and cylinders which improved cooling, and it had fewer parts than the Shovelhead which made it more reliable and easier to maintain.
The Evolution also had hydraulic valve lifters which eliminated the need for regular valve adjustments, and it had a stronger bottom end which allowed it to handle more power. Overall, the Evolution was a significant improvement over the Shovelhead and it became the standard motor for Harley Davidson motorcycles from 1984 to 1999.
In summary, the Panhead, Shovelhead and Evolution models succeeded the Knucklehead, each bringing new advancements and improvements in performance, reliability, and comfort. All these successors of the previous century are still cherished and sought after by enthusiasts today for their distinct design and HD heritage.
Knucklehead Restoration and Collectibility
Completely Original
Completely original 1937 Harley Davidson Knuckleheads are both rare and valuable. These machines have not undergone any significant modifications, making them highly sought after by collectors. The untouched nature of these bikes allows enthusiasts to appreciate the original craftsmanship and design.
Restored
Restored Knuckleheads garner a lot of attention in the motorcycle community. Many dedicated experts such as Carl Olsen of Carl’s Cycle Supply specialize in Knucklehead and Panhead rebuilds, ensuring that the bikes retain their authenticity while bringing them back to their former glory. Rebuild processes may include repairing cracks, cleaning oil return lines, and straightening or painting various parts.
Chopper Culture
The chopper culture also has an affinity for the Knucklehead. They were often used as the basis for custom 1960’s choppers, with their iconic design and strong performance capabilities. The influence of the Knucklehead on chopper design and the custom bike scene is undeniable.
Legacy and Significance
The Harley Knucklehead, introduced in 1936, marked a significant turning point in the history of the company. This innovative machine not only outperformed its main competitor, Indian, but also set new standards for reliability among American motorbikes.
In the world of American biker enthusiasts, the Knucklehead has become a symbol of engineering excellence and is much sought-after by collectors. The Wheels Through Time museum, for example, houses several rare editions of the Knucklehead, including a Teak Red, 61-cubic-inch version. Bikers who have had the privilege of riding a Knucklehead often attest to its smooth ride and impressive handling.
One of the major reasons for the Knucklehead’s lasting legacy is its groundbreaking design. With overhead valve V-twin technology it achieved greater power and improved the overall performance of their machines. This advancement allowed bikers to travel faster and farther, contributing to the enduring appeal of the brand.
The Knucklehead’s reliability also played a major role in its historical significance. HD from this era were known for their durability and longevity, giving bikers confidence in the brand. This reputation for reliability continues to hold true for both vintage and contemporary HD bikes.
In summary, the Harley Davidson Knucklehead holds an important place in biker history as a symbol of innovation, design, and reliability. Its introduction in 1936 forever changed the trajectory of Harley Davidson as a company and solidified its status as a leader in the American motorcycle industry.