There are icons in motorcycling, and then there are machines that sit in a category entirely of their own. The Honda CBX1000 has always occupied that space, its transverse inline-six defining both its engineering ambition and its unmistakable visual identity. For AC Sanctuary, a platform like this deserves an extra layer of detail, even above and beyond their usual fastidious approach. And with RCM-693, Hiroyuki Nakamura and his team have taken on the early-model CBX with a level of technical intent that goes well beyond surface-level modification. It truly defines the ‘Radical Construction Manufacture’ process, which gives their builds their name.
Built as a demonstration machine with full creative freedom, RCM-693 is the result of an internal dialogue between Nakamura and fellow builder Sasaki, with competing ideas resolved through engineering rather than compromise. Unlike their previous CBX-based RCM-688, which used a later model as its foundation, this early CBX presented a different set of challenges in geometry, packaging and base tolerances. The result is a machine that doesn’t lean on precedent, but instead establishes a new baseline for how the platform can be re-engineered.
Central to that rework is the chassis. To accommodate the move to 17-inch wheels, the rear suspension geometry has been altered via a precise laydown modification. Using a dedicated jig, originally developed for Kawasaki Z models and now commercialised through Noblest, the team achieved exact left-right alignment before fabricating a new upper shock mounting boss. This wasn’t a simple repositioning exercise; the change in angle necessitated a complete rethink of load paths and mounting hardware, eliminating the factory configuration entirely.
That philosophy continues with the swingarm. The new SCULPTURE unit is a monoblock design, integrating a ribbed square-section stabiliser into a single-piece structure. At the rear, the suspension linkage allows for adjustment through a 50-degree rotational range at the lower mount, giving a wide window for fine-tuning ride height and response. It’s a level of adjustability rarely seen on retro platforms, and it fundamentally changes how the CBX can be set up dynamically.
Rolling stock brings the package firmly into the modern era. The bike runs the latest GP-8R aluminium wheels from OZ Racing, here executed in a special RCM-specific 10-spoke configuration rather than the standard eight. Braking is handled by top-tier OMEGA discs from Sunstar, matched to contemporary caliper setups. The combination reduces unsprung weight while increasing rigidity and braking consistency, critical factors once performance begins to exceed original design limits.
The engine, however, remains the centrepiece. The CBX’s 1047 cc, twin-cam, 24-valve inline-six has been fully reworked by Dinks, the precision machining division within the Noblest group. This is not a cosmetic rebuild. The crankshaft undergoes dynamic balancing after journal lapping, while the cylinders are rough-bored and then finish-honed on a Rottler H85A to achieve exacting tolerances in both angle and clearance. Cylinder head work includes oversized valve guides and precision seat cutting, with all deck surfaces machined on NC equipment to ensure absolute flatness and sealing integrity.
That level of internal precision directly informs the external performance. Induction and exhaust are treated as part of the same system, with a 6-2-1 Weldcraft titanium exhaust from Nitro Racing positioned tight to the engine to maximise lean angle. The system is capped with the brand’s Swallow titanium silencer, finished in black. It’s worth noting that the exhaust routing isn’t just aesthetic; clearance and flow characteristics have been optimised to work with the engine’s revised tolerances and output.
Ergonomics and rider interface follow the same engineering-first approach. Rearsets are sourced from Nitro Racing’s dedicated CBX1000 line, while the seat is a one-off unit fabricated from A5052 aluminium sheet, cut, bent and welded to achieve a revised profile over stock. The subtle change in silhouette reflects a shift in rider position, aligning with the updated chassis geometry and intended use. Even the one-off V-shaped silencer, formed from bent aluminium in three dimensions, was planned from the outset as part of the overall package.
Despite being conceived as a demonstration build, RCM-693 didn’t remain in the workshop for long. After its debut at the Tokyo Motorcycle Show, the bike was quickly sold to a repeat RCM client with international racing credentials, raising the possibility that this meticulously engineered CBX may see circuit duty in some capacity. Whether or not that happens, the intent is clear: this isn’t a reinterpretation of a classic, but a comprehensive mechanical redevelopment, executed to a standard where every component is required to perform, not just exist.
[ AC Sanctuary ]