THE TALE OF THE LEGENDARY 250
Honda’s fabulous RC166 250cc Six was created because of the two-stroke. In 1964 the quarter-litre class was the most competitive in Grand Prix, with no less than 11 works teams and a host of fast strokers. The East German MZ and Yamaha’s RD56 had the reliability to win races, and though Suzuki’s square-four RZ63 had terrifying handling and broke a lot, Honda surely noticed its phenomenal speed. And then there was Bultaco, Aermacchi, CZ...
Having dominated the previous three years, Honda entered the ‘64 season confident they could keep ahead by updating the existing four-cylinder RC163 four-stroke. When Yamaha shattered this belief with Phil Read, Mike Duff and a fast, reliable two-stroke twin, Honda had to respond quickly. Its traditional method had become adding more, smaller cylinders for higher revs and increased power.
With the materials then available, the maximum mean piston speed Honda engineers could aim for in a engine capable of lasting a one-hour GP was 20.3 metres per second (these days even road bikes’ are 24m/s). The 250 four-cylinder’s 44 x 41mm dimensions already neared this figure and it was still getting beaten, so a shorter stroke and higher revs were out of the question. There was only one thing for it – build a six. Measuring 39 x 34.5mm, the 247.3mm six-cylinder wet-sump engine has one-piece barrels inclined forward by around 30˚, a one-piece head, and is, at 14in, no wider than the previous four. Double-overhead cams are gear-driven and made from two pieces, joined at the gear wheel, with four valves per cylinder and cylindrical followers. A built-up crank without counterweights gives two-stroke-like revving. There are six 17mm flat-slide Keihin carbs, three sets of points with three condensers, and three sets of coils. Early bikes rev to 17,000rpm, while later ones could hit 18,000.
A dry clutch with 16 plates and a sevenspeed gearbox transmits 60-odd bhp to the wheel. With a dry weight of 118kg (260Ib), power-to-weight bettered the best 500cc singles and gave a top speed of 153mph. In 297cc form, the six churns out over 65bhp without any increase in weight and could match even Ago’s 500cc MV Agusta.
A six-pot 250 would be pretty heady even today, but when Honda unveiled the RC166 on the first day of practice at Monza in September 1964 it stunned observers.
Honda took it there in great secrecy, even removing one megaphone on each side to make it appear it was nothing more than an updated 250 four should anyone peak under the cover laid over it from the moment it reached the track. It was one of the bestkept secrets in GP history – and kept Honda ahead of the chasing strokers.