The Honda RC166: six cylinders for 250 cm³, the acoustic masterpiece of Grand Prix racing

A six-cylinder in 250 cm³: Honda's crazy bet

In the mid-1960s, the 250 cm³ class was the technical battleground of the world championship. The Japanese two-strokes , led by the Yamaha RD56, imposed a specific output that four-strokes struggled to match. Honda, which refused the premix engine on principle, had only one way out: revs. And to raise the revs you have to shorten the stroke and cut the mass of the moving parts, so you multiply the cylinders. After the twins and then the fours, the Asaka race shop, in Saitamaprefecture, crossed the line of reason: as early as January 1964, a twenty-four-year-old engineer, Shoichiro Irimajiri, began work on a 250 cm³ straight-six. The same man would later design the 125 cm³ five-cylinder, then, much later, the production CBX.

The machine appeared at the 1964 Nations Grand Prix, at Monza, in September, in the hands of Jim Redman. Honda entered it under the misleading name 3RC164 and fitted only four exhausts in plain sight, to suggest a mere evolution of the four-cylinder. The ruse did not last, and neither did the race: faster than the Yamahas, the six was betrayed by vapour lock in its carburettors, and it was Phil Read who won on his RD56, taking Yamaha's first world title in the process. A few weeks later, at the Japanese Grand Prix at Suzuka, Redman finally won with the six. The line was launched: RC165 in 1965, then RC166 in 1966, the definitive version, stiffer and longer at its rider's request. Rarely has Honda in Grand Prix so openly embraced mechanical excess, in a world championship saga where the race for cylinders stood in for an arms race.

24 valves and 18,000 rpm: the anatomy of a UFO

The RC166's numbers are still hard to believe sixty years on. The engine displaces 245.6 cm³ with a bore and stroke of 41 x 31 mm : each cylinder therefore holds barely more than 40 cm³, the volume of a cigarette lighter. The engine is an air-cooled four-stroke, with double overhead camshafts driven by gears, and four valves per cylinder — 24 valves in total, tiny and therefore so light that they take accelerations unthinkable on a conventional engine.

That is the whole point of the formula: by splitting up the displacement, Honda cut the reciprocating mass and set the revs free. The RC166 delivers around 60 hp at 18,000 rpm, close to 244 hp per litre with 1966 technology, and tops 240 km/h at 112 kg dry. The crankshaft is press-assembled from thirteen pieces set on precision jigs, with crankpins of different diameters depending on the cylinder pairs to cope with torsional loads. Titanium and magnesium are used wherever possible, and six carburettors feed the whole thing.

A direct consequence of those revs: the usable band is narrow and sits very high. Hence a seven-speed gearbox and a dry clutch to keep the engine inside its window. The rest is almost disarmingly simple: tubular double-cradle frame, telescopic fork, two Girling shocks and drum brakes — 230 mm at the front, 200 mm at the rear, cooling fins included. All the cleverness sits in the engine, as it would later on the oval-piston Honda NR750.

The sound: the most famous acoustic signature in motorcycle sport

You can argue about race records, not about the noise. Six small cylinders spinning at 18,000 rpm produce an exhaust frequency that belongs to no other race bike: a continuous, almost musical scream, closer to a 1990s Formula 1 car than to a sixties two-wheeler. That signature has done more for the machine's legend than its statistics, and every public demonstration — especially at the big heritage gatherings such as Goodwood — turns into a sonic pilgrimage.

That red and silver silhouette, with its six megaphones lined up under the fairing, has become a picture-postcard image of motorcycle sport. Honda has since made a lasting selling point of it: the straight-six of the 1978 CBX 1000 descends in a straight line from that in-house obsession with a six-voice orchestra.

In the shop

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Hailwood 1966-1967: the perfect season and the most closely fought title

The RC166 could have been just an engineers' curiosity. It made history because Honda handed it to Mike Hailwood. In 1966, the Briton contested ten of the twelve rounds of the 250 cm³ championship and won all ten : riders' title, manufacturers' title, no argument possible. That same year, Honda pulled off a feat never repeated since, taking all five manufacturers' titles of the season, from 50 to 500 cm³ — a performance that has become impossible since the championship dropped to just three classes.

1967 was another matter entirely. Yamaha came back hard with Phil Read and Bill Ivy, and the duel turned into a brawl. At the end of the season, Hailwood and Read were dead level, 50 points each. The title went to the Honda rider thanks to a greater number of wins: five to four. The finish was in fact so confused that the FIM steward present in Japan, Otto Sensburg, refused to name a champion on the spot; the verdict, referred back to Geneva, first went to Read before being corrected in Hailwood's favour — the tie-break rule in force since 1965 appeared only in the French version of the sporting code, and not in its English translation.

In all, the RC166 gave fifteen Grand Prix wins to Mike Hailwood, on tracks as demanding as the Assen TT or the Isle of Man mountain course, where theTourist Trophy saga served as a global showcase for the Japanese six-cylinders.

The RC174: 297 cm³ to go and beat the 350s

The two machines are often confused, even though they did not race in the same class. The RC166 is a 250 ; the RC174 was Honda's weapon for the 350 cm³ class in 1967. The principle stays the same: keep the bore of 41 mm and lengthen the stroke to 37.5 mm, which brings displacement to 297 cm³ only. In other words, Honda went to take on 350s with 53 cm³ less than the maximum allowed, counting on revs to make up the difference. The RC174 claims around 66 hp at 17,000 rpm.

The bet paid off. In 1967, Hailwood won the 350 cm³ title with six wins and 40 points, ahead of Giacomo Agostini and his MV Agusta, credited with 32 points; Honda took seven of the eight races that season. The year before, the 350 crown had already gone to Hailwood, but on the RC173, a more conventional four-cylinder credited with more than 62 hp at 13,500 rpm and winner of six of the ten rounds: it really was the RC174 that brought the six-cylinder into the class. The lesson is crystal clear: in the 1960s, displacement mattered less than the number of revolutions per minute you could pull out of it without blowing up.

Honda's withdrawal and the end of an era

The 1967 peak was also a full stop. In February 1968, Honda announced its withdrawal from every class of the world championship. The reasons were many: development costs that had become dizzying, the priority given to cars and to Formula 1, and above all the preparation of an industrial revolution that would take the form of the 1969 CB750 Four. The brand paid Mike Hailwood out his contract — 50,000 pounds at the time — so that he would not race for any other manufacturer, hoping to have him back in the saddle on a future return. The Briton then turned to car racing.

The regulations finished closing the chapter. To contain the escalation and let privateers survive, from 1969 the FIM limited the 50 cm³ class to a single cylinder and the 125 cm³ class to two cylinders, then extended the two-cylinder rule to the 250 cm³ in 1970, with a cap of six ratios in the gearbox for every class. At a stroke of the pen, the RC166 and its seven-speed gearbox became illegal. Honda would not return to Grand Prix racing until 1979, with the improbable oval-piston NR500 , entered on 12 August at the British Grand Prix, at Silverstone. In the meantime, the 250 six-cylinder had stayed what it still is: a machine the rules now forbid anyone to copy, and whose soundtrack is its best argument.

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FAQ — Frequently asked questions

• What power and revs did the Honda RC166 really produce?
  About 60 hp at 18,000 rpm from 245.6 cm³, close to 244 hp per litre, with a top speed above 240 km/h for 112 kg dry.
• How many titles and wins did the RC166 take?
  Two 250 cm³ riders' titles with Mike Hailwood, in 1966 and 1967, along with the manufacturers' titles, and a total of fifteen Grand Prix wins. In 1966, Hailwood won the ten races he contested in the class, out of twelve rounds on the calendar.
• What is the difference between the RC166 and the RC174?
  The RC166 is the 250 cm³ (245.6 cm³, 41 x 31 mm). The RC174 is its offshoot, entered in 1967 in the 350 cm³ class: same 41 mm bore but stroke taken out to 37.5 mm, i.e. only 297 cm³ and about 66 hp at 17,000 rpm. The 1966 title-winning 350, the RC173, was a four-cylinder.
• Why did the RC166 have seven gearbox ratios?
  Because its usable power band, sitting very high in the revs, was extremely narrow. Adding ratios kept the engine around 18,000 rpm. The FIM capped gearboxes at six ratios and limited the number of cylinders from 1969-1970, making that solution illegal.
• Why did Honda leave Grand Prix racing at the end of the 1960s?
  Honda announced its withdrawal from every class in February 1968, because of development costs, the priority given to cars and to series production, and the FIM's tightening of the rules. The brand paid Mike Hailwood out his contract, 50,000 pounds, so that he would not race for any other manufacturer, and did not return to Grand Prix racing until 1979 with the NR500.