Honda Six-Cylinder RC174

The Rarest, Most Charismatic, and Historically Significant Motorcycle

When it comes to rarity, charisma, and historical significance, no other motorcycle can match the Hondas six-cylinder. While all the sixes were spectacular, the 297cc RC174 represented the culmination of Honda’s project and was one of the most successful. Only one original RC174 has survived, owned by my friend in Tokyo Terry Murayama. This has to be the ultimate collector motorcycle.

When the 250cc six was unveiled at Monza in September 1964 it created a sensation. It sounded like nothing else, and was blisteringly fast. Redman and chief mechanic Aika managed to get the RC165 to the Italian Grand Prix, flying it as hand luggage disguised under a blanket. To maintain its secrecy it was registered for the race as the 3RC164, with only four exhaust pipes fitted, the other two installed just prior to practice.

Although early testing in Japan showed the handling to be woeful, Redman hoped its impressive speed would psychologically unbalance Phil Read. He was right, Read was initially devastated. However the RC165 overheated in the race, causing the carburettors to vapour-lock and Redman finished third.

Continuing a philosophy of smaller cylinders and higher revs that was so successful on the 50cc twin, Honda’s engineer Shoichiro Irimajiri managed to create a six-cylinder engine with four-valves per cylinder that, at 356mm, was no wider than a four. With a bore and stroke of 39 x 34.5mm, the six followed the usual RC layout of a central train of spur gears driving double overhead camshafts and four valves per cylinder. The valves were set at a quite wide included angle of 75 degrees, the advantages of a shallower combustion chamber not yet appreciated. Its roller bearing crankshaft had no flywheels, so it stalled easily below 14,000rpm, and could break. The compression ratio of 12:1 was higher than usual for Honda racers. NGK supplied special 8mm spark plugs, and ignition was by magneto with three sets of contact breakers and the usual double-ended coils.

Carburetion was by six magnesium Keihin round or flat-slide carburettors, ranging in size from 17-22mm. The bank of carburettors was pre-set up on a bench, allowing easy installation at the track. The attention to detail and quest to reduce internal mass was staggering, with three different types of 70mm con-rod, the ones in the centre receiving larger big-end bearings where the load was higher. Ignition was by a gear-driven magneto, with three sets of points and three, two spark coils. According to Irimajiri, the power of the six-speed RC165 was 52 horsepower at 14,500rpm.

From the outset, the RC165 chassis was barely up to the task. Irimajiri admitted “in the late sixties we didn’t understand chassis rigidity.” The frame included the engine as a stressed member, the front fork featured welded steel sliders and the handling was poor. Braking was by a 220mm front, and 200mm rear twin leading shoe drum brakes.

For the 1965 season the 2RC165 theoretically should have had the better of the Yamaha twin but this was not to be. Now with a seven-speed gearbox (with many titanium components), Formula 1 commitments led to indifferent machine preparation and several gearbox failures. Redman was contesting the 250 and 350 titles alone. While he won the 350cc Championship, the 250 title was as good as lost by mid-season. That was despite three 250 victories, including another Isle of Man TT.

Honda then signed Mike Hailwood to ride the six. Hailwood loudly criticised the handling of the 2RC165, throwing the Japanese-made shock absorbers in a nearby lake after the first practice session. With Girlings fitted, Hailwood contemptuously ran away from the field on it in the final 1965 250 GP at Suzuka.

For 1966, Honda decided to contest every World Championship, aiming to repeat the domination of 1961. They needed another leading rider to share the burden with Redman, Taveri, and Bryans, procuring Mike Hailwood. Hailwood was undoubtedly the most brilliant rider of the era, but by the end of 1965 was beginning to tire of MV’s preference for their new Italian star, Agostini. So Hailwood signed for Honda for 1966, concentrating on the 250 and 350 titles. Thus began two magical years with Hailwood on the Honda six; possibly the best rider of all time, on one of the most charismatic racing motorcycles of all time.

After the disappointment of 1965, the 250cc RC166 was completely redesigned for 1966. There were new cylinders, and to improve cooling the cylinder head had additional fins and cut-outs. The sump had additional alloy fins welded on to it, and oil coolers were fitted in the fairing. The RC166 was now pumping out 56.8 horsepower at 17,500rpm. Hailwood requested improved handling, and the RC166 frame was built of stronger tubing (35mm at the steering head) with additional gussets under the fuel tank. The two-piece tubular steel swingarm included additional box-section steel bracing. There was also a new front fork, and the rider moved forward to take advantage of the improved grip provided by the new Dunlop triangular-shaped racing tyres.

The speed and reliability of the RC166 was faultless and the combination of Hailwood’s talent and the improved RC166 overwhelmed Read. Hailwood won all ten races he entered, absolutely blitzing the championship. While Hailwood took his second World Championship on the 350cc four-cylinder RC173 that year, there were already signs that the days of the four were numbered. Narrower and lighter, the MV 3 handled better, and despite a power deficit was close to matching the RC173. For 1967 Hailwood would have a six.

The 1966 racing season had been Hondas’ most successful ever, winning the manufacturers’ title in every category, but their increased involvement in Formula 1 car racing was stretching resources to the limit. For 1967, the motorcycle program was rationalised, with only two riders (Hailwood and Bryans) competing in three categories. All the expectations were on Hailwood, but it was too much to expect even his genius to win three classes.

Honda provided Hailwood with a new 250 six, the RC167, with a bigger bore and shorter stroke (41 x 31.5mm) and the output increased to 60 horsepower at 18,000rpm. There was an updated chassis, but the difficulties began with several retirements caused by engine failure. Ultimately Hailwood was declared champion because he had five race wins to Read’s four.

For the 1967 350cc World Championship Hailwood received the RC174 297cc six. This was a 250 six, enlarged as much as possible to 297cc (41x37.5mm). There were two 17mm intake and two 14.5mm exhaust valves per cylinder, and with six 24mm round-slide Keihin carburettors the power was 65 horsepower at 17,000rpm. While this wasn’t quite as much as the four-cylinder RC173, the RC174 weighed only 118kg (compared to the RC173’s 144kg) and was a considerably better balanced machine. Apart from generally running the larger RC181 500 four’s front brake, the RC174 was visually similar to the RC167.

The RC174 was always Hailwood’s favourite, and he started the season by absolutely crucifying Agostini at Hockenheim, winning by nearly a minute. Hailwood then went to the Isle of Man, winning the Junior TT at 172.8 km/h, with an absolute lap record of 173.34 km/h. Whereas the 250 six was problematic, the 297 was invincible. Further victories at Assen, Sachsenring, and Brno saw Hailwood clinch the World Championship with three rounds to go.

A change in FIM regulations for 1968 limiting the number of cylinders and stipulating six-speed gearboxes prompted Honda to withdraw from GP racing altogether. This coincided with the end of one of the golden eras of Grand Prix racing. In the space of nine years Honda had changed motorcycle racing, and emerged as the world leader in motorcycle technology. They had sold millions of motorcycles and were a household name. But they still hadn’t added the 500cc rider’s World Championship to their collection, and Honda would return. In the meantime their emphasis was on expanding the production range, with another milestone motorcycle, the CB750 four.

Photos courtesy of the Mick Woollett Archive