The Kawasaki GPZ900R is quite simply one of the most important motorcycles ever to be released in the history of two-wheels, and no, it’s not because Mr Cruise rode one in the original Top Gun movie. First launched in 1984, the GPZ was the world’s first true superbike, it brought with it the introduction of the 16-valve water-cooled inline four-cylinder engine that remains the dominate configuration in the category to this day. It was also the first bike to wear the Ninja name, so, you can understand why even four decades on, people still have a great love for the old Kwaka. At Japan’s AC-Sanctuary they have an entire series dedicated to the eighties ethereal warrior, and this build, RCM-606 is the latest to receive the full Ninja Formula Package with a little extra spice for good measure.

AC-Sanctuary might just have built more custom motorcycles than any other company on the planet, and they don’t just slap on a few bolt-on bits and call it a day. The top man at the company, Hiroyuki Nakamura understands just about everything there is to know about taking a Japanese bike of any age and capacity, and turning it into an incredible machine. His results speak for themselves and the owner of this bike not only has in his collection a number of other AC builds, but some of the most exotic cars and bikes ever to have been produced. So, when he placed an order for a GPZ900R, featuring the full Ninja Formula Package, he expected to receive something truly exceptional.

AC had already crafted nearly a hundred such builds based on the ’80s Kawasaki superbike, but Nakamura-san always has something extra up his sleeve. But first as is always the case, the frame goes into a jig to make sure it is 100% aligned and in excellent shape, with any welds needing to be repaired, completely ground out and redone. The GPZ was one of the first truly quick bikes not to have down tubes, utilising the engine as a semi-stressed member. But for a bike getting more power, bigger brakes and drastically increased grip, that needs to change. So a set of AC’s own Nitro Racing down tubes, in the type II configuration are bolted in.

Being of a machined aluminium construction, they can be tweaked to increase or reduce the exact amount of rigidity they provide, and this machine is destined for the street, and is not as stiffly set up as a track-orientated bike. A few more tweaks were needed to the frame, so as to accept Nakamura-sans surprise, a big block engine from the later GPZ1000. Adjustments are made to the mounts and swingarm pivot point, and these allow for the chain to properly align and for the wider tyre to be accepted. The block is then removed for the engine to be built up and the bike starts the process of becoming a roller.

At the rear end, the company’s signature Sculpture swingarm is deployed, and it’s all controlled by a remote res shock, built by Ohlins to an RCM specification. At the front end, a Sculpture steering stem kit helps the geometry to be precisely set, and then a pair of new triple clamps are machined up and anodised black to match the swingarm. Through these beefy alloy parts drops a set of Ohlins 43mm traditional way-up forks, and things have improved a long way from the old air-valve setup of the ’80s. To complete this part of the package, a full set of Brembo calipers are fitted, with AC’s own brake discs which are made for them by Sunstar.

The Oz-Racing wheels finish out the roller and become the first part of the visual transformation, and the lightweight 17-inch pieces are wrapped in Pirelli tyres, with a fat 190-section optioned for the rear. The bodywork is simply perfect, with many imitation stock parts used and made from lighter and stronger materials to replicate the factory tank, front fairing and tailpiece. The lower fairing and belly pan are left off, and the front fender is an in-house carbon item, with the stock offering simply too ugly to consider. Then it’s into the paint booth, for the amazing Ninja green and gloss black to be laid down, which is perfectly paired up with a smoked screen and Supreme seat.

Now the engine builders could bring the motor back into the assembly part of the factory, and a 4mm overbore sets the stage for some serious power. A set of larger forged pistons fill the holes and swing on an entirely balanced bottom end. The head has received the most work, with every port and passage flowed, the cams replaced and the entire valve train brought into the 21st century to accommodate big revs. To feed the fire, a set of Yoshimura FCR-MJN carbs with beautiful velocity stacks get the call-up, and the exhaust is the always incredible full Nitro Titanium system, which is built in-house for each particular bike.

The final piece to the puzzle saw Nakamura-san offer up a clever solution for his valuable customer, who wanted as much of a stock appearance as possible. So, rather than replacing the electronics with superior modern items, everything including the gauges, switchblocks and other parts, has been stripped and rebuilt with entirely new components on the inside.

The result is a machine that functions flawlessly and yet still has the look of the original. A few compromises had to be made however, with a new ignition system fitted to handle the power and a new cooling system simply proving the must-have for long term reliability, especially when riding hard. Complete with the big brother engine, perfect Ninja paintwork and all the goodies, RCM-606 is a more than fitting homage as the GPZ fast approaches its 40th birthday. 

[ AC Sanctuary ]