By 1964 Honda's four-cylinder RC164 had reached the limit of its development, and Yamaha's two-stroke RD56 had proven faster. The company charged 24-year-old engineer Shoichiro Irimajiri with designing a replacement. The core challenge was fundamental: two-stroke engines fire on every crankshaft revolution, while four-strokes fire every other revolution. Honda's founder Soichiro Honda had committed publicly to four-stroke engines on grounds of refinement and efficiency, so the answer was to rev the engine far beyond what rivals could manage with two-strokes.
Irimajiri's solution was a six-cylinder layout with four-valve heads, spreading reciprocating mass across six cylinders to allow extreme revs while maintaining reliability. The design was completed in early 1964 and a running engine produced by June of that year. The resulting engine was no wider than the four-cylinder it replaced — a remarkable packaging achievement.
The first six-cylinder machine, designated 3RC164, was readied in time for the September 1964 Nations Grand Prix at Monza. Honda shipped it disguised with only four exhausts visible to mislead competitors. The machine was faster than the Yamahas but suffered a carburettor vapour lock, and Jim Redman could not finish. Redman won on the 6 at the following Japanese Grand Prix at Suzuka, providing the first victory for the concept.
An updated version, the RC165, was introduced early in the 1965 season. It used an 8-speed gearbox and produced 56 bhp at 16,500 rpm from a 39 mm bore and 34.8 mm stroke. Jim Redman's season was repeatedly disrupted by injury — a fall at the Nürburgring in the 350 cc race prevented him starting the 250 race in round two, and a broken collarbone in Ulster cost him further rounds — and he ultimately competed in only six of the season's championship rounds. He won three, finishing third in the championship. Honda came second in the constructors standings.
Late in 1965 Mike Hailwood signed with Honda for the 1966 season. After winning the season finale Japanese Grand Prix on the RC165, Hailwood was forthright in his criticism of the machine's handling, requesting a stiffer, longer-wheelbase frame and better suspension components ahead of the following year.
For 1966 Honda produced the RC166, incorporating the revised frame Hailwood had demanded. With the improved chassis the machine's handling transformed. Hailwood won all 10 of the races he entered during 1966, taking the 250 cc riders' championship and Honda the constructors' title. Ralph Bryans partnered Hailwood in 1967 as Yamaha responded with a newly engineered water-cooled 250 V4. The two manufacturers traded victories across the season, and Hailwood and Yamaha's Phil Read finished level on points at the year's end; Hailwood took the championship on countback by virtue of five wins to Read's four. Honda again secured the constructors' championship.
The RC166's inline-six displaced 245.6 cc from a bore and stroke of 41 mm by 31 mm. The block was cast as part of the upper crankcase, with cylinder heads cast in two halves covering three cylinders each. Titanium and magnesium castings were used extensively throughout. Camshaft drive ran through a spur gear train between cylinders 2 and 3; camshafts were barrel-shaped with greater mass at the centre to resist flexing. The crankshaft was assembled from 13 separate components pressed on precision jigs, with different-sized crankpins at different positions to manage torsional loading. Drive to the clutch passed via a jackshaft geared between cylinders 3 and 4.
The engine produced 60 bhp at 18,000 rpm and used a 7-speed gearbox (8-speed on the RC165) with multi-plate dry clutch and chain final drive. Front brake was a 230 mm four-leading-shoe drum, with a 200 mm two-leading-shoe unit at the rear.
At the end of 1967 the FIM introduced regulations for 1968 limiting the number of cylinders and gears in each class, deliberately constraining the exotic multi-cylinder configurations that Honda had pioneered. Honda announced its full withdrawal from Grand Prix racing. Paddock rumour held that a water-cooled V8 had been planned as the RC166's successor. Honda presented Hailwood with a RC166 in recognition of his two 250 cc championships on the model.
The 350 cc RC174, a bored-out version of the RC166 producing 297 cc, also raced in 1967; Hailwood won its first five races to clinch the 350 cc championship as well, giving Honda a remarkable double in their final full season of 1960s Grand Prix competition.