Honda had first entered Grand Prix motorcycle racing in 1959 and by 1962 was dominating the 250 cc class. In 1964, however, the existing inline four-cylinder RC164 was outpaced by the Yamaha RD56, and Honda recognized the platform had reached its development ceiling. The company initiated work on an entirely new design to restore its dominance.
The engineer placed in charge of the project was Shoichiro Irimajiri, then just 24 years old. Design work began in January 1964 and initial sketches were completed by February, with a running engine produced by June of that year.
The central engineering challenge was power density. Two-stroke engines used by competitors Suzuki and Yamaha fire every crankshaft rotation, while four-strokes fire every two rotations. Soichiro Honda had publicly committed to four-stroke engines for their cleaner, quieter, and more efficient character. Irimajiri's solution was to add cylinders — six in total — with four-valve heads, reducing reciprocating weight while allowing the engine to rev high enough to produce competitive power. The resulting six-cylinder was no wider than the four it replaced and narrower than Yamaha's twin.
Designated 3RC164 to suggest to competitors it was merely an evolution of the existing four, the bike debuted at the September 1964 Nations Grand Prix at Monza, shipped with only four exhaust pipes visible to further disguise its configuration. Despite being faster than the Yamahas, a vapour lock in the carburettors cost victory, with Phil Read winning on the RD56 to clinch Yamaha's first world championship. The 3RC164 then won the following Japanese Grand Prix at Suzuka with Jim Redman.
An updated version, the RC165, was introduced early in the 1965 season. The year proved difficult — Redman suffered injuries in the 350 cc race at the Nürburgring that cost him several rounds, and gearbox failures interrupted other results. Redman ultimately finished third in the championship despite competing in only six rounds.
Late in 1965 Mike Hailwood signed a contract to return to Honda for £40,000. After winning the season finale Japanese Grand Prix on the RC165, Hailwood was candid in his criticism of the machine's handling. He reportedly had the rear suspension units removed and thrown into a pond, requesting Girling units as replacements, a stiffer frame with a longer wheelbase, and a new chassis built by Dutchman Nico Bakker — an outside involvement Honda declined.
For 1966, the RC166 was introduced with the stiffer, longer frame Hailwood had demanded. The handling improvement was decisive. Hailwood and the RC166 were dominant throughout the season, winning all ten races he entered, though championship points counted only the best seven results. Honda also won the constructors' title.
In 1967 Ralph Bryans partnered Hailwood on the RC166s. Yamaha had developed a water-cooled 250 cc V4 in response to the Honda 6 and re-engineered it for reduced size and weight. At season's end Hailwood and Yamaha's Phil Read were tied on points; Hailwood took the championship by virtue of his five wins compared to Read's four. Honda again secured the constructors' title.
At the end of 1967 the FIM introduced new engine regulations for the 1968 season — limiting cylinder count and reducing the number of gears — measures that levelled the playing field in favour of European manufacturers. Honda withdrew from Grand Prix racing rather than adapt. Paddock rumour held that the RC166 would have been superseded by a new water-cooled V8 had the regulations not intervened.
Honda presented Hailwood with an RC166 in recognition of his two world titles on the machine.
The RC166's six-cylinder engine was engineered to extraordinary precision. The block was cast as part of the upper crankcase; the heads were cast in two pieces, one for each bank of three cylinders. The engine made extensive use of titanium and magnesium castings and was capable of revving beyond 18,000 rpm.
The crankshaft was pressed up from 13 separate components on precision jigs. To reduce the effective vibrating length of the crankshaft, drive to the clutch was transmitted through a jackshaft driven by a gear located between cylinders 3 and 4. Different sized crankpins were used to manage torsional forces: largest for cylinders 3 and 4, smaller for 2 and 5, and smallest for 1 and 6. Connecting rods were one-piece with roller bearings.
Camshaft drive ran via a train of spur gears between cylinders 2 and 3. The camshafts were barrel-shaped with greater mass at the centre to minimise flexing. Each cylinder carried a four-valve head, with each of the four valves a different shape.
Bore and stroke were 41 mm × 31 mm, giving a displacement of 245.6 cc. Power output was 60 bhp at 18,000 rpm, transmitted through a multi-plate dry clutch, a seven-speed gearbox, and chain drive.
The cycle parts used a duplex cradle frame, stiffer and with a longer wheelbase than the earlier RC164 and RC165 frames. Front forks were telescopic; the swingarm was controlled by two Girling shock absorbers. Braking was by drums — a 230 mm four-leading-shoe unit at the front and a 200 mm two-leading-shoe at the rear, both fitted with cooling rings. Wheels were 18-inch, shod with Dunlop K124 tyres.
In 1966, Hailwood had complained about the handling of the 350 cc four-cylinder RC173. For 1967, Honda responded with a larger-capacity derivative of the RC166, the RC174, with stroke increased to 37.5 mm for a displacement of 297 cc. Hailwood won the first five races of the 1967 350 cc season, enough to secure the championship. Bryans rode the RC174 for the remaining rounds and finished third in the final standings.