If Harley-Davidson has a "holy grail" engine, the EL Knucklehead is undoubtedly it. Produced between 1936 and 1947, the Knucklehead— named for the distinct shape of its rocker boxes—represented a massive technological leap for the Milwaukee firm. It was Harley’s first production overhead-valve (OHV) V-twin, a design that replaced the aging "flathead" and set the template for every Big Twin that followed.
For custom builders, the Knucklehead is the ultimate status symbol. Whether it’s a genuine 1940s survivor or a high-performance modern reproduction from S&S Cycle, the engine’s architecture is inherently beautiful. It’s a powertrain that demands to be showcased in a rigid frame, stripped of excess, and treated with the reverence usually reserved for cathedral architecture. Here are five of our favorite Harley-Davidson Knucklehead customs featured on Bike EXIF.
‘The Disciple’ by American Metal Customs
Josh Allison of American Metal Customs built ‘The Disciple’ based on a genuine 1941 Harley-Davidson EL engine. This bike is a study in mixed-metal fabrication. The most striking element is the bodywork, which takes direct inspiration from a 1950s Schwinn Hornet bicycle, complete with bronze "art-deco" wings on a chiseled split tank.
Josh didn’t stop at the tank; the saddle is a complex assembly of steel and bronze layers held together with contrasting rivets and a hinged front mount. To keep the stance authentic, he used a reproduction straight-leg rigid frame and "windowed" the neck for a cleaner look. The front end is a shortened Indian-style leaf spring setup, which provides a plush ride while keeping the 21-inch front spool wheel tight against the frame.
Despite its vintage appearance, ‘The Disciple’ is built to be ridden. It features a Baker 6-into-4 transmission (giving the rider six gears instead of the stock four) and dual rear calipers for modern stopping power. It’s a bike so evocative that American Metal even launched a whiskey line aged using the acoustics of this very Knucklehead engine. [MORE]
‘Jenny Lind’ by Asterisk Custom Works
Japan’s Mooneyes show is the ultimate proving ground for craftsmanship, and Hideki Hoshikawa of Asterisk Custom Works arrived with a masterpiece. Named after the Swedish opera singer Jenny Lind, this bike was built entirely from scratch over the course of a year. There is no "donor bike" here; Hideki used basic hand tools to shape a one-off chromoly frame with a unique single down-tube design that mimics the curve of the S&S crankcase.
The "Chevy Bel Air" influence is visible in the fuel tank and side panels, which feature trim concepts taken from the 1957 American classic. To maintain a "sano" cockpit, Hideki used internally routed throttle cables and a foot-clutch/hand-shifter setup. The front suspension is a modified leaf spring setup from Cannonball, paired with a massive 23-inch front wheel for a dramatic, spindly profile.
Every detail is a nod to traditional techniques. The frame was chrome-plated and then scuffed with Scotch-Brite for a satin sheen, while the exhaust is a stubby two-into-one system designed to stay out of the frame’s way. ‘Jenny Lind’ is a high-water mark for the Japanese custom scene, blending mid-century Americana with obsession-level metalwork. [MORE]
The Zero-Style Knuck by Rough Crafts
Winston Yeh of Rough Crafts is known for his "Guerrilla" style, but for this build, he leaned into the "samurai chopper" aesthetic pioneered by Shinya Kimura. Using a Zero Engineering Type 6 frame as the skeleton, Winston dropped in a 93ci S&S Knucklehead motor to create a machine that feels both Victorian and gothic. The bike's stance is defined by the Zero springer forks and a "neo-vintage" mix of brass and black finishes.
The fabrication is where Winston’s genius shines. He created custom weld-on hand controls with a remote hydraulic master cylinder and a hidden lithium-ion battery box to keep the lines as clean as possible. The sheet metal is a highlight, featuring the signature Rough Crafts scalloped tank and a minimalist solo seat that sits low in the frame’s crook.
While many Taiwan-based builds are purely for show, Winston’s "Brass Racer" aesthetic is backed by the stump-pulling torque of the S&S mill. It’s a perfect bridge between the traditional Japanese "Zero" style and the modern, aggressive street-fighter vibe that has made Rough Crafts a global powerhouse. [MORE]
The Sculptured Knuck by Matt Machine
Australian builder Matt Machine created this sculptured metal masterpiece for the prestigious Born-Free show in California. His brief was to combine a rigid frame with high-end modern components, essentially creating a "best of" album of motorcycle engineering. He paired a 93ci S&S Knucklehead with a Baker 6-speed gearbox and a massive pair of 40mm Dell’Orto carbs.
The mechanicals are a global affair: Brembo brakes from a Ducati 999, a Moto Guzzi master cylinder, and Kawasaki ZXR inverted forks held in polished custom triples. The contrast between the high-performance sportbike front end and the classic V-twin heart is jarring but perfectly executed, thanks to the cohesive hand-beaten alloy bodywork.
The finish is a soft, satin sheen achieved by blocking back the alloy fairing and tank with multiple grades of Scotch-Brite. Every detail, from the wooden crate built for its flight to Sydney to the custom switchgear from Purpose Built Moto, screams precision. It is a retro-futuristic vision of what a Harley-Davidson can be when the constraints of the "cruiser" label are completely ignored. [MORE]
The ‘Vintage Racer’ by Custom Works Zon
Yuichi Yoshizawa of Custom Works Zon threw the custom world a curveball with this "aged" racer. While the engine is a modern S&S KN93 producing 83 lb-ft of torque, the bike looks like it was recently unearthed from a 1930s barn. The cylinder heads were modified to accept a pair of Linkert M88 carburetors mounted on handmade manifolds.
The frame is a custom steel unit that is four inches shorter than a stock FL frame, giving the bike an incredibly compact, aggressive silhouette. The wheels are particularly unusual: 18-inch rims with hollow pipe spokes attached to custom-machined hubs. Yoshizawa-san even went as far as to dent the fuel tank and "soot up" the white high-temp paint on the exhaust to simulate decades of track abuse.
Everything on the bike, from the hill-climb-style springer forks to the weathered race seat, was aged by hand using gasoline streaks and light rust. It’s a "restomod" in reverse—a high-performance machine wearing a grimy, monochrome mask that Yoshizawa recently put to the test at the Chirihama Sandflats beach races. [MORE]
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