It was once the fastest production motorcycle money could buy, designed by a Formula 1 engineer and with a power plant that Jay Leno describes “as still the sexiest engine out there.” But the Honda CBX1000 was never a sales success, and despite some tweaks during its five-year production run to make it even better, Honda was practically giving them away in the end. For the general public, this masterpiece of automotive design was simply too complicated, 24 valves, six cylinders and individual carbs were all too much. Fast forward decades later and if you’d bought as many new examples at those giveaway prices as you could, you’d be a very rich man. The world has finally come to appreciate why the CBX is so great, but that complexity is still there. And despite normally being able to turn out a radical resto-mod in a matter of months, this AC-Sanctuary build number RCM-688, took the Japanese outfit over three years to finish, but boy was it worth the wait.
Long-time readers of Pipeburn will know that I am an unapologetic fanboy of AC-Sanctuary and the legend that is its founder and lead engineer, Hiroyuki Nakamura. He’s spent decades making perfection look easy, and has a backlist of customers desperate to pay big money for one of his builds; but he’s the first to admit that RCM-688 caused him to sweat through his overalls and suffer many a sleepless night. And once again it was the complexity of the monster motor’d Honda that caused all the stress. This was the first time that Nakamura-san had used a CBX1000 for a full RCM build, so when the order came in from CH Moto in Taiwan, the conversation was as much about how long the build might take, as much as any particular features the client, Mr Chen, might desire.
To commence the build, a later model Pro-Link equipped example was found and the team at AC quickly had it torn down, so they could examine every part and piece. Not having an existing jig on which to build the chassis, a new one had to be fabricated, and with the unusual frame bolted in, work commenced on converting it into a 17-in wheel compatible, street missile. Using the engine as a stressed member, there are no down tubes or lower loops on the frame to help support that super heavy engine. So, a considerable amount of work was poured into strengthening the neck and swingarm pivot point, so that the increased load generated by the new horsepower, braking and handling package, could be well controlled.
Even with all of this work carried out and rigorously tested by the in-house engineering team, Nakamura-san spent extra time ensuring the new steering stem was up to the task and that extra gussets were placed in all of the vital areas. In fact, when it came time to bolt on the stunning all-aluminium, hand-crafted swingarm, the decision was made to make the main bolt from Chromoly. And keen eyes will also notice that while the original donor bike used a single shock setup, the team reverted back to a twin shock rear end, with the upper mounts seriously beefy, strength wasn’t to be compromised. As always, the shocks are fully adjustable Ohlins items and the front end is more of the Swedish goodness, with gold USD forks clamped by in-house CNC machined triple clamps.
The fork lowers are more of the company’s stunning custom work and allow the big Brembo radial calipers to mount up, which along with the large floating discs help to provide the sort of stopping power a bike of this size needs. To further assist in that regard and lighten up the handling too, ultra-lightweight Oz Racing wheels go on, and considerable work was needed to allow for the 190-section rear tyre to fit. The rearsets were another challenge, with Mr Chen wanting to ride two-up, so the guys made all billet items, with the heel guards serving as the mounts for the rear pegs. The dash too is all handcrafted, a mix of carbon and alloy, which has its own mounts and doesn’t turn with the bars, supporting a full set Stack gauges.
But at the heart of it all is that engine, the six-cylinder beast took up a huge part of the build, and it’s here you see Nakamura-san’s commitment to perfection. Assembled in the company’s DiNx engine processing facility, lightweight and excellent balance were top of the list. The crank has been partially knife-edged, the journals all retreated and the assembly fully balanced. The pistons are lightweight forged DiNx items as are the factory processed rods. But ARP in the US was employed to make the nuts and bolts, and the first set was sent back. The rod cap bolts had not achieved the exact 0.127mm of stretch Nakamura-san had calculated should be the number when torqued down. This is vital for ensuring excellent clamping without deforming the journals, the second set caught a jet to Japan and all tightened up to exacting specs, and the stretch gauge showed they were bang on to a thousandth of a millimetre.
The top end is all hand-built using NOS Honda components and a host of custom-made DiNx parts, the goal being to achieve a light and reliable valve train, rather than hunting for big power. “The Nitro Racing titanium exhaust and mufflers are a one-off masterpiece. At the owner’s request, it is an elaborately made muffler suitable for the RCM, equipped with a silencer on both sides.” But before this bad boy could sing like a Formula 1 car, and it does, the process of fitting up the six individual Keihin FCR33 carbs had to be done. Then they had to be tuned, ensuring perfect balance across each cylinder and allowing the bike to idle smoothly, make masses of torque and then rev to the moon!
The bodywork is about as extravagant as we’ve seen from AC-Sanctuary, with Mr Chen wanting his CBX to stand out from the crowd. But it strikes a perfect mix of classic lines and captivating bling, with the custom graphics laid down over a traditional Honda colour scheme and all of the lines look like they come straight from the factory. Even the big ’80s tail light remains in place, but the seat has been built from scratch to give the rider the ultimate in grip and comfort, and carbon mirrors and front fender help to keep the weight under control. Nitro bars, Brembo masters and new switchgear finish out the build, and the boys have been invited to exhibit the bike at next weekend’s Tokyo Motorcycle Show. Then it’s on a boat bound for Taiwan, and it is hard to imagine any machine that will have the road presence, the rev-filled roar or the riding experience of the remarkable RCM-688.
[ AC-Sanctuary ]