1940 Harley-Davidson E and EL Knucklehead: the Final 61ci-Only Prewar OHV Big Twin

The 1940 Harley-Davidson Knucklehead occupies a precise and important place in Milwaukee history. It was not the first Knucklehead, and it was not the largest, but it was the last prewar model year in which Harley-Davidson’s overhead-valve Big Twin line was built around the 61 cubic inch engine alone. One year later, the 74 cubic inch F and FL joined the range, and the character of the OHV Big Twin family broadened from a relatively high-performance 61 into the larger-displacement format that would dominate postwar American heavyweight motorcycling.

For collectors, the 1940 E and EL sit at an appealing crossroads: later and more developed than the difficult first-year 1936 machines, still unmistakably prewar in trim and engineering, and historically separate from the 1941-47 74ci Knuckleheads that followed. It is a motorcycle for buyers who understand why model year, displacement, crankcase identity, and original equipment matter as much as polish and paint.

Best Known For: the 1940 Harley-Davidson Knucklehead is best known as the final 61ci-only prewar OHV Big Twin year, immediately before the 74ci F and FL changed the Knucklehead family in 1941.

Quick Facts

The table below separates the facts that define the 1940 E and EL from broader Knucklehead folklore. Exact production totals and some equipment combinations are not consistently documented across surviving references, but the mechanical identity of the 1940 61ci OHV Big Twin is well established.

In collector language, the key phrase is 61ci Knucklehead. That distinguishes the 1940 E and EL from the 1941-47 74ci OHV models and from Harley-Davidson’s side-valve U-series Big Twins, which can look superficially similar to casual observers but are mechanically and historically different machines.

Why the 1940 61ci Knucklehead Matters

The 1940 Knucklehead matters because it shows Harley-Davidson’s OHV Big Twin after several years of development, but before wartime disruption and before the large-displacement 74ci OHV models arrived. The 1936 EL had announced Harley-Davidson’s new overhead-valve direction with ambition and some well-known early teething troubles. By 1940, the design had matured into a more dependable and commercially credible performance motorcycle.

It also matters because 1940 is a dividing line. Enthusiasts often group all Knuckleheads together, but a 61ci prewar machine is a different historical object from a post-1941 74ci FL. The smaller engine, prewar equipment, rigid frame, spring fork, hand-shift controls, and period civilian presentation make the 1940 E and EL especially interesting to restorers who want a motorcycle that still feels rooted in the late Depression era rather than the immediate postwar boom.

Historical Context and Development Background

Harley-Davidson entered the late 1930s with Indian as its principal American rival and with heavyweight motorcycles serving a mixture of private, police, commercial, sidecar, and long-distance uses. The side-valve Big Twins remained important to Harley-Davidson’s business, but the overhead-valve E-series was the company’s technical statement: faster breathing, better performance potential, and a more modern image for the premium solo rider.

The Knucklehead nickname came later from the shape of the rocker covers, whose lobed ends suggested clenched knuckles. Factory literature did not present the motorcycle under that name when new, but the term is now indispensable in collector and restoration circles. It distinguishes the 1936-47 OHV Big Twin from the later Panhead and Shovelhead engines whose rocker covers gave rise to their own nicknames.

By 1940, Europe was already at war, and American manufacturers were watching military requirements closely. Harley-Davidson’s eventual wartime volume would be dominated by the 45ci WLA rather than the Knucklehead, but the broader market was changing quickly. Civilian production, police purchasing, export availability, and material priorities would all be affected as the United States moved closer to direct involvement in the war.

Against that background, the 1940 61ci Knucklehead is the last full expression of the pre-74ci OHV Big Twin idea. It was a prestige road motorcycle, not a mass-produced military tool, and its appeal was based on performance, modernity, and Harley-Davidson’s belief that the overhead-valve Big Twin had become essential to its future.

Engine and Drivetrain

The 1940 E and EL used Harley-Davidson’s 61 cubic inch air-cooled OHV V-twin, a 45-degree engine with pushrod-operated overhead valves, cast-iron cylinders, and aluminum cylinder heads. The visible rocker boxes are the feature that gave the Knucklehead its later nickname, but the more important engineering change was the move away from the side-valve breathing of earlier Big Twins. Better cylinder filling and more efficient combustion made the OHV motor a sharper and more sporting proposition than the flathead U-series.

Fuel was delivered by a Linkert carburetor, with ignition by a battery-and-coil electrical system typical of Harley-Davidson practice of the period. The rider managed spark advance manually, a normal feature on American motorcycles of the era and a crucial part of starting and low-speed running. Lubrication was dry-sump, with an external oil supply and engine-driven oil circulation rather than the primitive arrangements associated with much earlier motorcycles.

Power reached the rear wheel through a chain primary drive, a multi-plate clutch, a four-speed transmission, and a rear chain final drive. The hand-shift gearbox and foot clutch are central to the machine’s character. They are also central to restoration accuracy, because later controls, converted clutches, and custom-era modifications can change both the riding experience and the collector value.

Documented mechanical specifications are best kept separate from later performance claims. The table below covers the core engine and driveline architecture without relying on disputed speed or output figures.

The E and EL distinction is commonly understood in terms of compression and tune, with the EL representing the higher-performance 61ci version. For identification and valuation, however, the stamped model prefix, original crankcase integrity, and correct period equipment matter more than casual claims about output.

Chassis, Suspension, and Braking

The 1940 Knucklehead used Harley-Davidson’s Big Twin rigid chassis with a spring fork at the front and no rear suspension beyond the saddle, tires, and frame flex. This was normal for American heavyweight motorcycles of the period. The later Hydra-Glide telescopic fork and swingarm rear suspension belonged to a different chapter of Harley-Davidson history.

The visual stance is a major part of the 1940 machine’s appeal. The engine sits exposed and architectural in the frame, with the rocker boxes breaking up the vertical line of the cylinders. The tanks, tank-top instrumentation, valanced fenders depending on equipment, and broad prewar road presence give the motorcycle a very different look from the stripped postwar bobbers that later used Knucklehead engines as raw material.

Braking was by mechanical drums front and rear. For its day, the layout was ordinary and serviceable; by modern standards, stopping power is limited and highly dependent on cable or rod condition, drum condition, shoe material, and careful setup. A properly restored brake system is a safety issue, not a cosmetic detail.

Wheel and tire specifications can vary by equipment, market, and surviving-machine history, and many 1940 Big Twins have been altered over decades of use. Serious judging or restoration work should be based on period Harley-Davidson parts books, factory literature, and marque-specialist review of the individual motorcycle.

Riding Experience and Mechanical Character

A 1940 Knucklehead is not ridden like a modern motorcycle with nostalgic styling. The starting ritual demands that the rider understand fuel, choke, throttle opening, spark position, and kickstarter technique. When cold, the machine rewards method rather than force; when warm and correctly tuned, the OHV twin should answer with the heavy, uneven cadence that made the Big Twin such a distinctive road engine.

The control layout defines the experience. The right hand manages the throttle, the left hand is involved with spark control, the left foot operates the clutch, and the rider shifts through a tank-side gate by hand. A stock rocker clutch is not the same thing as a later custom suicide clutch, but it still requires anticipation in traffic and a calm left foot at stops.

On roads of its era, the 61ci OHV motor gave the rider a stronger and cleaner pull than Harley-Davidson’s older side-valve machines of similar displacement. It is not merely a slow antique if assembled and tuned properly, but the pleasure is torque, pulse, and mechanical involvement rather than high rpm. The gearbox has long, deliberate movements, the clutch rewards careful adjustment, and the driveline makes its work audible.

The rigid rear frame and spring fork give the motorcycle a planted, heavy feel at speed and a very direct relationship with the road surface. Broken pavement comes through the chassis and saddle rather than being isolated by suspension. The brakes require planning, especially downhill or in traffic, and the rider quickly learns to use engine braking, spacing, and mechanical sympathy.

Identification and Originality

Correct identification begins with the engine, because pre-1970 Harley-Davidsons were identified primarily by the engine number rather than a modern frame VIN system. A 1940 E or EL will be associated with a 40E or 40EL model prefix on the engine number pad, followed by a production sequence. The exact sequence should be evaluated carefully by a specialist, because restamped cases and altered number pads are among the most serious issues in the Knucklehead market.

Frames from this period do not carry a matching VIN in the modern sense. Collectors therefore pay close attention to crankcases, belly numbers, casting details, machining, fasteners, and the consistency of the motorcycle as a whole. A beautiful restoration built around suspect cases is not the same market object as a motorcycle with credible original cases and period-correct supporting parts.

The most common originality problems are not subtle. Many 61ci Knuckleheads were enlarged, updated, chopped, bobbed, or rebuilt with later parts during their working lives. Later 74ci flywheels or top-end components, postwar sheetmetal, later tanks, later wheels, non-period handlebars, converted controls, updated lighting, and reproduction forks or fenders can all appear on otherwise attractive machines.

Correctness is also visual. A proper 1940 machine should present as a prewar Big Twin, not as a postwar Panhead wearing Knucklehead cases or a custom-era chopper returned to stock-looking paint. Tank-top instruments, period-style hardware, correct finishes, cadmium or parkerized details where appropriate, and the relationship between engine, frame, tanks, fenders, and fork all matter in serious judging.

Documentation is valuable but must be read critically. Old titles, prior registrations, restoration invoices, period photographs, and marque-club judging sheets can help establish continuity. They do not automatically prove that a set of cases, frame, and sheetmetal began life together, but they can make a strong motorcycle much easier to trust.

Model Code and Variant Breakdown

The 1940 61ci Knucklehead range is most commonly discussed through the E and EL model codes. Police, export, and special-equipment machines are best treated as specifications or orders rather than as wholly separate 1940 OHV engine families unless documentation for the individual motorcycle proves otherwise.

This distinction is central to the 1940 motorcycle’s identity. A 1940 machine represented as a 61ci E or EL should not be quietly treated as equivalent to a later 74ci FL, even if it has been modified internally or externally over the decades.

Performance and Dimensional Specifications

Period and later sources often discuss the 61ci EL as a strong-performing prewar American heavyweight, and many references associate the high-compression EL with roughly 40 horsepower. Because output depended on compression, fuel, tune, measurement method, and source, it is better to treat horsepower as a model-context figure rather than a universal specification for every surviving 1940 E or EL.

Likewise, published top-speed, weight, and dimensional figures vary across sources and machine configurations. Equipment, police or touring accessories, wheel and tire choice, and restoration decisions all affect the numbers that buyers encounter. For a serious purchase or judged restoration, the reliable numbers are the mechanical architecture, displacement, bore and stroke, model code, and documented equipment rather than modern performance summaries.

Compared With Related Harley-Davidson Models

1940 E / EL vs. 1936-1939 Knucklehead

The earlier Knuckleheads carry first-series appeal, especially the 1936 EL as the debut OHV Big Twin. They also carry the burden of early-development scrutiny. By 1940, Harley-Davidson had refined the OHV package, making the motorcycle attractive to riders and collectors who want prewar character without concentrating solely on first-year rarity.

1940 E / EL vs. 1941-1947 F / FL Knucklehead

The 1941 arrival of the 74ci F and FL changed the family. The later 74s are often favored by riders who want the greater torque and the familiar FL identity that carried into later Harley-Davidson history. The 1940 61ci machine, by contrast, is desirable precisely because it predates that expansion and preserves the earlier displacement identity.

1940 Knucklehead vs. U and UL Flathead Big Twins

The U-series side-valve Big Twins were robust and well suited to sidecar, police, and utility service, but they do not deliver the same OHV mechanical identity as an E or EL. The Knucklehead’s exposed rocker architecture, higher-performance image, and position as Harley-Davidson’s premium OHV road motorcycle make it a different collector proposition.

1940 Knucklehead vs. 45ci WLA Military Harley-Davidson

The WLA belongs to the military 45ci side-valve family, not the Knucklehead family. It is historically important for wartime production and military use, but it should not be confused with the 61ci OHV Big Twin. The 1940 E and EL are civilian premium motorcycles at the edge of the wartime period, not mass-produced military workhorses.

1940 Harley-Davidson Knucklehead vs. Indian Chief

The Indian Chief was the principal American heavyweight rival, but it was a side-valve machine with a different engineering philosophy and road character. Indian emphasized smoothness and a strong large-displacement flathead personality. Harley-Davidson’s 61ci OHV Knucklehead offered a more modern valve train and a sharper performance image in the premium solo class.

Restoration and Ownership Notes

Restoring a 1940 Knucklehead is not simply a matter of finding shiny parts. The difficult work is establishing what the motorcycle is before money is spent: correct cases, credible numbers, proper frame, correct fork, appropriate tanks and sheetmetal, and a coherent set of period components. A machine assembled from expensive reproduction pieces can be beautiful and usable, but it will not occupy the same collector tier as a well-documented original or an accurately restored motorcycle built around major original components.

Parts availability is better than for many prewar motorcycles because the Knucklehead has long been supported by specialists, reproduction manufacturers, and marque enthusiasts. That abundance is a mixed blessing. It makes restoration possible, but it also means incorrect or newly made components can be fitted easily and then aged, painted, or described optimistically.

Engine rebuilding requires a specialist who understands Knucklehead oiling, rocker gear, case integrity, crankshaft assembly, cam and breather timing, and the effects of previous enlargement or repair. Cracked or welded cases, damaged number pads, mismatched case halves, worn cam bushings, poor oil return, and incorrect rocker-box work can turn an attractive purchase into a very expensive education.

Ownership also demands sympathy. A properly rebuilt 1940 E or EL can be a usable vintage motorcycle, but it must be operated within the logic of its materials, brakes, tires, cooling, oiling, and controls. Frequent inspection, clean oil, correct adjustment, and restraint at sustained high speeds are not optional rituals; they are how these machines survive.

Buyer and Restoration Inspection Points

The following inspection points reflect the issues that most affect authenticity, cost, and long-term satisfaction. They are not a substitute for a Knucklehead specialist’s inspection, but they frame the questions a serious buyer should ask before committing.

The best 1940 Knucklehead purchases are rarely the ones with the loudest claims. They are the motorcycles whose physical details, documentation, and mechanical condition all tell the same story.

Collector and Market Relevance

The collector appeal of the 1940 Knucklehead rests on three pillars: prewar manufacture, 61ci OHV identity, and placement immediately before the 74ci FL era. It has the mature look and usability of a later prewar Knucklehead without losing the smaller-displacement character that defined the original E-series concept.

Rarity is difficult to express responsibly because exact production splits and survival numbers are not consistently documented. What is clear is that correct, well-documented 1940 E and EL machines are much less common than assembled, modified, or partially restored examples. The market responds accordingly, with authenticity, crankcase integrity, correct sheetmetal, and quality of restoration carrying heavy weight.

The Knucklehead also has an unusually strong custom-culture shadow. From postwar bobbers to later choppers, the engine became one of the most desirable foundations for American custom building. That cultural importance helped keep the Knucklehead visible, but it also explains why many original motorcycles were altered, cut, or parted out.

Collectors typically value a 1940 example for what has not been changed. Original cases, correct 61ci configuration, credible E or EL identity, prewar chassis and sheetmetal, and period-correct finishes matter more than showy chrome or non-original upgrades. A restored motorcycle can be excellent, but a restoration that erases evidence or hides questionable components will always invite scrutiny.

Cultural Relevance

The 1940 E and EL were not factory race specials in the way Harley-Davidson’s dirt-track and Class C machinery were. Their relevance is broader: they helped establish the OHV Big Twin as Harley-Davidson’s premium road-engine format. That mattered deeply to the brand’s later identity, even though wartime production would temporarily steer the company toward military 45s.

Police departments and service riders valued Harley-Davidson Big Twins, and some 61ci OHV machines were equipped for official or special-duty use. Still, the 1940 Knucklehead should not be treated as a military motorcycle unless a specific machine has documentation proving a service connection. Its core identity is civilian and premium rather than utilitarian and standardized.

In club and collector culture, the Knucklehead remains one of the defining American engines because it combines visible mechanical drama with historical consequence. The 1940 model year adds a sharper point to that appeal: it is the final stop before the 74ci OHV Big Twin becomes part of the story.

FAQs

What makes the 1940 Harley-Davidson Knucklehead different from other Knuckleheads?

The 1940 model year is the final prewar year in which the OHV Big Twin Knucklehead line was 61ci only. In 1941 Harley-Davidson introduced the 74ci F and FL, which changed the Knucklehead range and created the larger-displacement FL lineage that collectors often associate with postwar Big Twins.

Was the 1940 Knucklehead an E or an EL?

Both E and EL model codes are associated with the 1940 61ci OHV Knucklehead. The EL is commonly identified as the higher-performance or higher-compression version, while the E represents the standard 61ci OHV model. The stamped engine prefix and supporting evidence should be checked carefully on any individual motorcycle.

How do I identify a real 1940 Harley-Davidson E or EL Knucklehead?

Start with the engine number pad, which should correspond to a 1940 E or EL identity, commonly using a 40E or 40EL prefix. Then evaluate the crankcases, belly numbers, frame, fork, tanks, transmission, controls, and period equipment. Because pre-1970 Harley-Davidsons were identified primarily by engine number, restamped or questionable cases are a major concern.

Is the 1940 Knucklehead a 61ci or 74ci motorcycle?

A correct 1940 E or EL Knucklehead is a 61 cubic inch motorcycle. The 74 cubic inch OHV F and FL models were introduced for the 1941 model year. Some surviving 61ci machines have been modified internally, so displacement should be verified if originality matters.

Was the 1940 Knucklehead used by the military?

The 1940 Knucklehead was principally a civilian premium Big Twin. Harley-Davidson’s best-known wartime military motorcycle was the 45ci WLA side-valve model, not the Knucklehead. Any claim of military use for a specific 1940 E or EL should be supported by documentation.

Are parts available for a 1940 Knucklehead restoration?

Yes, specialist support and reproduction parts availability are comparatively strong for the Knucklehead family. The challenge is not merely finding parts, but finding correct parts and knowing which original components are worth saving. Reproduction cases, sheetmetal, fork parts, and hardware can affect collector value even when they improve usability.

What hurts the value of a 1940 E or EL Knucklehead?

Questionable engine numbers, restamped cases, mismatched or heavily repaired crankcases, chopper-era frame alterations, incorrect 74ci conversions, later sheetmetal, non-period controls, and undocumented restoration work all hurt value. The strongest motorcycles have credible identity, correct major components, and documentation that supports the physical evidence.

Collector Takeaway

The 1940 Harley-Davidson E and EL Knucklehead deserve attention because they mark the last clean year of the original 61ci OHV Big Twin concept before the 74ci FL broadened the family. That makes them more than merely another prewar Harley. They are the mature version of the motorcycle Harley-Davidson introduced in 1936 to prove that its future did not have to remain side-valve.

For the serious collector, a 1940 Knucklehead is about restraint and evidence. The right machine is not the one dressed up to look expensive; it is the one whose cases, frame, engine specification, controls, and prewar equipment still support its claimed identity. In a market full of restored, reassembled, and modified Knuckleheads, a correct 61ci 1940 E or EL has a historical clarity that cannot be added later with paint.