Honda’s RC166 was a 250cc Grand Prix motorcycle with a four-stroke inline-six specified to produce more than 60 PS at 18,000 rpm. Honda describes its six open exhaust pipes as “music to race fans of the 1960s,” but the available historical sources do not establish a decibel level or verify that spectators cringed or covered their ears.
What was the Honda RC166, and how loud was it?
Introduced for the 1966 season, the RC166 was Honda’s six-cylinder entry in the 250cc Grand Prix class. Its engine speed and unusual exhaust arrangement help explain why its sound became part of its reputation. Honda Motor Europe later called the engine’s sound through its six open pipes “music to race fans of the 1960s” (Honda Motor Europe).
That is an attributed, subjective description—not a sound measurement. The cited historical accounts provide no sound-pressure figure, test distance, venue measurement or named witness for the claim that people cringed or covered their ears. The bike’s high-rev specification is documented; that specific crowd reaction is not.
RC166 specifications
Honda’s historical specifications describe a compact, high-revving four-stroke built for Grand Prix competition. These are manufacturer-published figures, not independent test results.
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| Specification | Honda’s figure |
|---|---|
| Engine | 249.42cc, air-cooled, four-stroke inline-six |
| Valve gear | Double overhead cams; four valves per cylinder |
| Maximum output | Over 60 PS at 18,000 rpm |
| Transmission | Seven-speed gearbox |
| Weight | 112 kg |
| Maximum speed | Over 240 km/h |
Honda lists these figures in its RC166 historical specifications and Japanese RC166 specification table. The published “over” figures should be read as manufacturer specifications, not as a promise of a particular result under every track or operating condition.
Why build a six-cylinder 250?
Honda says it developed the six-cylinder RC166 as two-stroke rivals became stronger in the 250cc category (Honda’s RC166 history). The six-cylinder layout was part of a competition response: Honda pursued an engine that could operate at very high speed, rather than treating the extra cylinders as a novelty on their own.
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That design also gave the bike a distinctive exhaust note. Honda’s account connects the sound to the six open pipes, but it does not quantify loudness. Without a documented measurement, comparisons such as “painfully loud” cannot be turned into a factual decibel claim.
How successful was the RC166 in Grand Prix racing?
Honda’s history credits Mike Hailwood with ten wins from ten races entered in the 1966 season. In 1967, Honda reports five victories for Hailwood and two for Ralph Bryans, while the manufacturer retained both rider and manufacturer titles (Honda Japan’s road-racing history; Honda Global’s RC166 history).
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The ten-from-ten figure refers to races Hailwood entered, as Honda states it; it should not be recast as a season-wide percentage with a different denominator. The published histories describe title-winning campaigns, not a complete independent race-by-race results audit.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Build a scale model
For readers interested in the machine as a model, Tamiya documents a 1/12-scale Honda RC166 GP Racer plastic assembly kit, item 14113. It is a kit that requires assembly, not a ready-built motorcycle (Tamiya product information).
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