It all started early in 2025 with six American builders and concluded on the otherside of the Pacific when the S&S Cycle Vintage Tour rolled into Pacifico Yokohama, for the massive Mooneyes event. Three Japanese builders had been chosen, and among the very select handful picked to represent S&S’s legacy was a stark, purposeful, knuckle-powered board tracker from Infinity Motorcycle. A bike that looked less like a showpiece and more like something that had just rolled off the timber boards, still warm from competition, dripping with custom cool and celebrating more than a century of that spectacular sport.
Infinity Inc. is based in Iruma City, Saitama Prefecture, where the shop quietly balances modern Harley-Davidson service work with some of Japan’s most historically informed custom builds. Founded by Masumi Tsuchino, Infinity’s roots are grounded in mechanical discipline. Before turning to motorcycles, Tsuchino cut his teeth in the automotive world, earning a level-one mechanic certification with Toyota. That foundation still shows today: precision first, aesthetics second, and history always respected.
The bike itself is called BT6, a name that’s refreshingly literal. It’s the sixth board tracker Tsuchino-san has built, and arguably the most conceptually ambitious. Commissioned directly by S&S Cycle, the brief wasn’t to recreate a Harley-Davidson or Indian racer, but to imagine an alternate timeline, one where S&S existed in the 1920s and went toe-to-toe with the American giants on the board tracks of the era. It’s historical fiction, executed in steel and aluminium.
At the heart of BT6 is a KN93 S&S Knucklehead engine, finished with modified surface treatments to push it closer to period-correct visuals. Displacing 93 cubic inches (1524cc), the motor runs a 3.625-inch bore and a 4.5-inch stroke, with an 8.2:1 compression ratio. Inside are S&S forged pistons and an S&S 420 camshaft, while fueling comes via an S&S Super E carburettor. Spark duties are handled by the S&S Super Stock electronic ignition, modern reliability discreetly hidden within a vintage silhouette, and packing a big punch.
The chassis follows the same philosophy. A replica 1936 Knucklehead frame was modified to suit the board track layout, stretched and pared back to its bare essentials. Rolling stock comes from The Cyclery, with clincher rims sized at 3×22 up front and 3×20 at the rear, reinforcing the bike’s early racing stance. Up front, a set of vintage girder forks, their exact origins lost to time, have been carefully modified and brilliantly reengineered to work in harmony with the rest of the build.
One of the more subtle modern concessions is the Baker Drivetrain 6-into-4 complete transmission, which, just like the engine, packages modern tech inside a classic look. It’s a reminder that while BT6 looks like it belongs to another century, it was built to operate reliably in this one. The paintwork, handled by Rio Studio and New Classic Painting, is restrained and period-sensitive, allowing the form of the machine to do the talking rather than relying on colour or gloss for impact.
The relationship with S&S was central to the build. As Tsuchino explains, BT6 was created as a gesture of gratitude, an opportunity to showcase S&S components within a style Infinity knows intimately. That sentiment was echoed by S&S Cycle owner and director Brett Smith, who noted how meaningful it was to return to Yokohama as part of the 2025 Vintage Tour, building on decades-long relationships within the Japanese custom scene.
BT6 isn’t trying to rewrite history, and it’s not chasing nostalgia for nostalgia’s sake. Instead, it sits quietly in the space between fact and fiction, asking what might have been if S&S had been there at the dawn of American motorcycle racing. In Masumi Tsuchino’s hands, that question is answered with a masterpiece that blends show levels of fit and finish, with a bulletproof drivetrain that packs power and reliability. Stripped of excess and rich in intent, BT6 feels less like a modern custom and more like a recovered artefact. And to really appreciate this brilliant build, you need to take the time to study each and every photo; she’s one tasty knuckle sandwich.
