6 THERAPY!
Allen Millyard’s latest project extends his engine reconstruction skills further by building a six-cylinder version of Kawasaki’s iconic Z1. John Nutting finds out what it’s like to ride
If you’ve been following Allen Millyard’s exploits with his project to build a six-cylinder version of the Z1 Super Four, Kawasaki’s answer to Honda’s CB750 launched in 1972, you must have been wondering just what it’s like to ride.
This saga, the latest in a number of epic projects to build what many riders regard as unfeasibly complex machines that look like the result of a wild and untempered imagination, started at the Stafford Show back in October 2019.
At the show, Kawasaki parts specialist Dave Marsden had reminded Allen that he should build an in-line six, no doubt thinking that having already turned out a V8 based on the Z1 and a V12 in the Z1300, it would be a relatively straightforward task.
Now, Allen’s never one to resist a challenge and at first he considered building another V8, thinking a six might be too wide to make a convincing example of what Kawasaki might have produced. This prompted the acquisition of parts from a 1976 Z900 engine, but while studying these in his workshop, Marsden’s idea came to mind.
Allen made some paper templates to show how an inline six might look, and what the width would work out at.
Because Allen has been a master of metal since he started his craft apprenticeship at the turn of the 1980s, he’s a bit old money and rightly so, so the measurement came out at around 24 inches. He wondered how this compared with the daddy of six-cylinder bikes, Honda’s CBX from 1978, and recalling that I’d mentioned being at the bike’s launch in Japan, he sent me an email to ask if I remembered how wide it was.
Width was a key issue with the CBX and Honda’s engineers had done a great job in minimising it, so I knew that it was 23½ inches across the cases, because I’d measured one. That reassured Allen, and the project was off the mark.
The fabrication, however, wasn’t as straightforward as it had been with Allen’s RC374 six-cylinder race replica based on two Yamaha FZR250 engines. Although Allen’s videos of the process of cutting and machining the engine castings make it look easy, if laboriously timeconsuming, they don’t show the amount of calculation necessary before those first hacksaw cuts are made.
So it was almost Christmas before he really started to make progress with the reconstruction. The cases had to accommodate two extra outer crank throws while the cylinder block and head uses the two inner pots, with two added either end, the idea being that the overlapping structure, like a brick wall, is more robust.
Key during the welding of the cases was to minimise distortion by aligning the main-bearing supports with a large mandrel and initially heating the parts in the barbeque to 150 degrees C before making initial tack welds and then filling in. It’s wonderful to see in the videos how, rather than machine the joint faces, Allen masterfully hand files the welds down and then laps them with a cast-iron block to a smooth grey. More than three decades of experience in engineering and metalwork enable Allen to make it look like we could all manage it.
The crankshaft was assembled by retaining the two inner throws and hydraulically pressing the four outer ones (two of which required additional weights welded on) at 120-degree intervals to give theoretically perfect balance.
All the original roller main bearings had been retained because despite being more than 40 years old they had barely any wear.
Amazingly, because the engine was built up with 1mm oversize (67mm) pistons giving a swept volume of 1396cc, the valves – all 12 of them – lapped and the clearances set, the transmission and clutch assembled with slightly heavier springs, it took just five weeks before everything was in place.
Rather than adapting the original contact breakers for the ignition, a one-off electronic system was ordered from Boyer Bransden, the Kent-based specialist that kindly supplied it free of charge. On January 27, the Super Six was fired up for the first time.
What was now needed was a rolling chassis to slot the engine into. Neil Howarth, who does all of Allen’s paintwork, had amongst his bike collection an unrestored Z900A4 without an exhaust system that fitted the bill. Because the Super Six engine retained the original mounting points, it slotted straight into the Z900A4 chassis, which was restored with Neil’s painting skills. The final touches were the addition of intake plumbing for the outer two carburettors, 28mm Mikunis as on the original Z1.
Allen prefers a look that appears to be factory-fresh. An attempt to adapt three stock silencers either side didn’t work, so stainless steel collectors were made to feed the three pipes either side into the original exhaust system. From the side you have to look hard to notice the difference between a standard Z1 and Allen’s Super Six. And no better tribute came from the man responsible for styling the Z1, Norimasa ‘Ken’ Tada, who sent Allen a message of approval.
The complete bike was run for the first time on March 22, about three months after the engine had been started, just as lockdown was announced to prevent spread of the coronavirus. It took another six weeks for Allen to properly test ride the Super Six, mindful that at the time taking it out would give a bad impression for motorcycling, of riders enjoying themselves while others were in lockdown in their homes (which I agree with).
In the meantime Allen took advantage of being confined to the workshop by starting work on a second Super Six engine, this time using 70mm pistons from the Z1000, and giving a bigger capacity of 1524cc. “It’s all ready to fire up on its test stand, but I haven’t had time to organise it with my filming commitments with Henry Cole and my YouTube channel,” says Allen. “I’m in no rush and aim to complete the second bike mid 2021.”
By then the word had got around, and Allen says he has been bombarded with offers to buy replicas. Allen has such a high media profile, what with appearing on television programmes like The Motorbike Show and Find it, Fix It, Flog It, along with producing his own YouTube videos, that it’s no surprise his fan base wants more. But that’s for the future, perhaps. There was still the task of fine-tuning the Super Six before Allen let me loose on it.
Once he’d got the bike out on the road, Allen reckoned it felt like it was geared too low, which with the standard rear sprockets (15 tooth front and 35 tooth rear giving about 130mph in top) would be no surprise. Allen fitted a larger 16 tooth engine sprocket and then reduced the rear to 29 teeth, the smallest he could fit, giving a lazy 3500rpm at 70mph.
Stickler that he is, Allen wanted
the bike as good as he could get it, so corrected one or two small problems, such as with one of the idling jets, which meant it wasn’t until September that the time came for the bike to be ridden by someone other than Allen and Henry Cole, who took a short spin for the television cameras.
For me, the key issue with the Super Six would be how it compares with a stock Kawasaki Z1. The last time I’d ridden one of the original bikes was back in 1973 a year after its launch and while the memory of its performance was vivid, the context of the time was more hazy. I’d reported then that the Z1 ‘runs very smoothly’, but that was in comparison with the big twins and triples of the time. The smoothest bikes on the market were the shaft-drive boxer twins from BMW, regarded as a wealthy gentleman’s machine and beyond the means of most riders.
So to provide a more realistic yardstick against which to compare the Super Six, Allen arranged for Neil to bring along a very tidy Z1 dating from 1974 that he’d owned for many years. The Z1 had been called New York Steak by Kawasaki’s American marketing people, and they weren’t kidding. Neil’s bike felt raw but chewy by today’s more refined standards, with a buzz through the handlebar – characteristic of a 180-degree in-line four – complemented by the whine of the straight-cut primary gears. Throttle response was abundant, particularly so from low revs, when it was enough to get the huge 630 final drive chain rattling in harmony.
I’d forgotten how light the Z1’s steering could be at speeds below 50mph, the front end dancing around unless you took firm control of the high and wide handlebar. At higher speeds the bike would be more secure in its navigation, but never quite as planted as I would have liked.
Now it was time to ride the Super
“Now it was time to ride the Super Six, a moment I’d been anticipating with relish ever since Allen had first considered the idea, barely 10 months earlier.”
Six, a moment I’d been anticipating with relish ever since Allen had first considered the idea, barely ten months earlier.
You don’t appreciate the extra width until you’re sitting astride the Super Six. The riding position is no different from the stock Z1 – you can sit astride the bike, boots flat on the ground – apart from intrusion of the inlet tubes to the outer two carburettors to the inside of your knees. But the motor is huge, wonderfully so. It fires up on the button with the warbling growl typical of a six, and enhanced by slightly larger and less restrictive outlets from the silencers.
On the broad stock handlebar the controls are silky, but fractionally heavier; stronger clutch springs are detectable and the open-close throttle cables resist the more tightly wound return springs on the carbs. But pulling away from a standstill is no effort despite the higher gearing; bottom would be good for more than 60mph if I’d been careless of Allen’s request not to rev the nuts off it.
Not that there’s a need. With six cylinders there’s no sense of the engine being lugged and it pulls smoothly all through the range, significantly more smoothly than I’d expected. There’s a flood of torque even though I was keeping to a 5000rpm limit, which if we’d been on an open road would have been good for the ton in top gear. The engine’s smoothness is such that the drive chain’s slight vibration, accentuated by the smaller sprockets, intrudes. Out of curiosity I trickled down in top to see how low it would pull from and it was 1500rpm, but at these revs it was the chain that baulked at the task, chattering against the swingarm.
But I was being perverse. Out on twisty lanes in Berkshire I was surprised that the Super Six felt more balanced than the original Z1, the front end more planted. And though I wasn’t about to explore the cornering limits of the modern Avon Roadrider MkII tyres, or for that matter risk decking the crankcases, the Super Six offered a sense that its handling envelope was much wider.
Curious about the effects of the additional weight of the engine on the front-end – along with the additional brake disc – I’d asked Allen if he could put each end on some scales. Dutifully, he stole some from the bathroom and came up with 299 pounds front and 308 pounds rear, or a total of 607 pounds or 275kg. Sounds a lot I know, but referring back to my test notes from 1975, the Z900 weighed in at 250 front and 300 rear, for a total of 550 pounds (250kg) fully tanked up.
So, as you’d expect, most of the extra weight of the Super Six engine is on the front wheel, but it hasn’t harmed the bike’s behaviour at all; in fact the weight improves it.
Apart from using marginally heavier damping oil, Allen hadn’t changed the springing of the front fork either. He says this is common to all of his Kawasaki specials with extra cylinders – up to five on the two-strokes – in that the additional weight keeps the front-end better planted.
There’s still some fine tuning to be carried out. I found the carburation a bit woolly at just over idle and Allen says that a session with vacuum gauges will probably improve it. Even so the fuel consumption is meagre, he says.
As with all of Allen’s bikes, the detailing on the engine is exquisite. You’d need forensic skills and intimate knowledge of Kawasaki fours to identify where the joins in the crankcases are located, mostly because the finish has been so well homogenised.
The cover for the ignition on the offside is slimmer, but the opposite side’s generator cover is unchanged, although Allen considered removing the starter motor to save width – the engine is so easy to turn over on the kick lever it wouldn’t have been missed, Allen says. The gap under the starter’s cover has been filled with aluminium sheet so well you can barely see the difference.
Unlike many wild specials that shout their excesses, Allen’s Super Six follows many of his projects, being subtle and almost understated. His objective again is to produce a machine that onlookers will think came from the factory.
The Super Six is extra special though. It succeeds in that, and works better in our classic world than the Z1 it was derived from.