AC Sanctuary Builds the Ultimate Air-Cooled Inline-Fours

Words: Roland Brown
Photography: AC Sanctuary

At first glance these could be just faithfully restored classic bikes—big Japanese machines with a few modifications that were common during the late-’70s/early-’80s heyday of the naked superbike. But look closer and you see these air-cooled inline-fours are much more than just restored old machines. Modified frames, intricately welded titanium exhausts, high-tech suspension and fat, 17-inch tires confirm that these are highly modified specials combining classic lines with modern superbike technology.

Welcome to the high-octane, nostalgia steeped world of AC Sanctuary, the Japanese tuning shop that creates these stunning “resto-mod” versions of classic Japanese superbikes. Founded in 1995 by air-cooled Kawasaki super-enthusiast Hiroyuki Nakamura, the shop has surfed the wave of nostalgia that has earned these early Japanese fours a rabid cult following in their home country. Demand became so great that AC Sanctuary soon began selling complete bikes under the name RCM, for Real Complete Machine.

AC Sanctuary now employs 20 people between its three-story headquarters in Edogawa, eastern Tokyo, as well as six branch locations scattered around Japan, and the firm has built more than 150 RCM bikes to date. Most are early Kawasakis, primarily mid-’70s Z1s and more angular KZ1000s from the early ’80s. Other popular models include Suzuki’s GS1100 and Katana, along with Honda’s twincam CB900F and CB1100F.

No matter what the starting point, AC Sanctuary follows the same basic format for all RCM builds. Engines are stripped and comprehensively rebuilt, often with Wiseco or Yoshimura pistons to increase displacement and raise compression. Crankshafts are balanced, gearboxes blueprinted and heavy-duty camchains and clutches inserted. The fnishing touch is usually a curvaceous exhaust from AC Sanctuary’s in-house Nitro Racing parts line, welded from heat-treated titanium and capped with a diminutive, free-flowing muffler. Typical output is more than 100 bhp. Combined with a substantial weight reduction, RCM bikes deliver acceleration that the

[Nakamura prefers the stock bodywork and paint, like on this Honda CB1100F, to emphasize the retro appeal. The titanium exhaust is pure 21st century, however.

original owners could only dream of.

Similar improvement is realized by chassis modifications. Steel frames are stripped and strengthened with gussets atthe steering head, then rewelded with steeper geometry. A plate added in the swingarm-pivot area allows the drive chain to be moved outwards to clear a wider rear tire. Most are fitted with a billet aluminum swingarm from Japanese aftermarket frm Sculpture; some heavily braced, others deceptively ordinary-looking, yet still much stiffer than stock. Brakes are typically from Brembo or Braking; wheels are often lightweight 17-inchers sourced from Marchesini or O.Z., wrapped in fat Pirelli or Dunlop rubber.

All bikes are built to order, so final specifications vary according to customer preference. Fit-and-finish and detailing are invariably show-quality, whether it’s the refurbished standard components or modern parts such as custom-made wiring looms, aluminum rearsets, anodized fork braces or racer touches including safety-wired brake caliper hardware. The original bodywork is almost always retained, with paintwork in respectfully standard style, offset by a black-finished engine.

[The Suzuki Katana's distinctive, wedge-like fairing is as close as you'll come to seeing bodywork on any RCM creation. Pumped-up engines are always the focal point

Whichever the model, the result is a visual treat for fans of classic superbikes. The best thing about AC Sanctuary’screations, however, is that these rejuvenated old warriors are genuinely great motorcycles to ride, with performance that’s radically improved over the originals. It’s no wonder the company has a constant waiting list, despite the inevitably high price of machines requiring many hundreds of hours’ work. And although a genuine Real Complete Machine is financially out of the question for most Japanese enthusiasts, these bikes surely inspire many less ambitious projects along similar lines. The quality and attention to detail make each RCM bike utterly unique, but the basic appeal of a hotted-up classic superbike is appealingly universal.