In other words, apart from a few nuts and bolts, every single one of the engine's hundreds of components would have to be made from scratch. And not a single one of them, according to Julian Charnol, the man responsible for engine assembly, is anything less than extraordinary. Little wonder JPX regards the RC174's Six as "the most difficult engine we have ever seen ... so full of hidden complexity."
"When they analyzed the engine," Beale recalls, "the pattern-makers were amazed-really impressed by the advanced techniques Honda must have had to make it, which few other companies could have done." Yet at the time it designed the first Six (the RC164 250), Honda had been racing internationally for just five years.
Above all, there was the Six's crankshaft-beautiful, but a bitch to re-create. By '64, Honda had already licked the technology to get the most out of inline-four engines. Adding two additional cylinders might not sound such a big leap forward, yet the implications for engine assembly are huge. The Six's crankshaft is pressed up from 13 components, each no bigger than a domino. Unsupported, it is so flimsy it can be deformed by hand, yet it would have to spin without deflecting at more than 17,000 rpm. Pressing it together with the necessary accuracy-0.01 degree-would require an elaborate set of jigs weighing more than the complete bike; if even one part became slightly misaligned, the entire assembly would be scrap. Ludovic Surcin, designer of the jigs, likened the task to balancing 13 billiard balls on top of each other-and persuading them to stay put.
At one stage it was hoped to use a bulletproof modern crank with split con-rods, but there was simply no room inside the Six's tiny crankcases to accommodate the extra bulk. Consequently the replica retains a design exactly like the original's. With almost no flywheel effect to slow it down, one careless blip of the throttle can send the crank from zero revs to more than 20,000, whereupon it would simply break.
Other details emerged that showed the remarkable lengths to which the Six's design team had gone to reduce the mass of moving components. JPX was astounded to discover three different types of con-rods in each engine, with progressively larger big-end bearings for the rods nearer the center, where loadings are higher. Even 40 years ago the benefit of such a strategy was well-known, but no one else bothered to take advantage of it. Any other manufacturer would have made six identical rods, each able to cope with the highest possible loading. But Honda's obsessive desire to have no component heavier than it need be caused the engineers to tailor each pair precisely for their function.
In the same way, the seven main bearings are also different, ranging from 24mm in diameter at the center to 14mm for the outer pairs. The crankshaft itself runs on carriers bolted to the top crankcase half, which takes all the loads. Thus the bottom half can be made much lighter and flimsier from magnesium alloy.
Similarly, the camshafts (each head has four, linked together in pairs) are barrel-shaped, concentrating their mass at the point of greatest flex, the center, and thus saving weight at their ends. But this in turn means the geometry of the valve gear varies from one cylinder to the next, with different cam profiles. The benefit is tiny, the resulting difficulties immense, yet at every turn Honda went to such obsessive lengths seemingly regardless of cost.
Other than the crankshaft, JPX's biggest challenge was in replicating the Six's tiny but elaborate castings at its in-house foundry. The cylinder block, for instance, is sand-cast using a three-dimensional jigsaw of no less than 29 interlocking sub-molds (known as cores). Casting the heads requires 23, while even the relatively simple sump needs 15.
Even with the very latest computer-controlled technology, machining the major castings was even more complex. JPX's CNC machine-as big as a truck-took several weeks to set up for the job. The machining operation itself took 14 hours and 90 tool heads to machine a single cylinder block. How Honda engineers of the early 1960s did this simply staggers the mind.
The engine is riddled with galleries and tiny oilways, unseen by anything but X-ray, with some only 1mm apart. Sometimes two narrow oilways are combined, saving perhaps 0.5mm in width on a single, larger one. Cumulatively this allows the engine to be perhaps 3mm narrower than it might otherwise-yet another minute but worthwhile return on the work involved.
The tiny pistons are machined from solid, and run in iron cylinder liners. Tolerances are so fine the liners must be dipped in liquid nitrogen before being pressed into the pre-heated block. To reduce internal friction, no oil control rings are fitted, just paired compression rings. As with a few other highly stressed components, JPX had the wrist pins coated with a space-age diamond-like finish, the hardest surface treatment known to science. At the other extreme, many of the new engine's gaskets were simply cut out using a pair of scissors.
Nothing so crude would do for the carburetors, replicas of Honda's round-slide Keihins. The floats are individually modified from those of a current Honda road bike, and even the jets had to be specially machined. Each bank of six cost more than $35,000 to make.
The Six is littered with bearings. All but one or two are needle, ball or roller, which offer less drag than plain bearings. At one stage during the project JPX had one man who did nothing but contact bearing manufacturers all over the world trying to source parts which might fit the Six's bespoke demands. After several weeks he had located perhaps one-third of the team's needs. The rest, including bearing cages, had to be laboriously made from scratch.
The more you delve into the Honda Six's inner secrets, the more you marvel at the skill and confidence of the men who first built it, almost four decades ago. "Compared to this," Beale sighs, "the Benelli was built by a blacksmith."