In 1965, Honda used an inline six cylinder motorcycle to contest the 250 cc motorcycle Grand Prix World Championship. It was built at the time as a viable solution to compete against similar displacement two stroke power plants.
As was covered in this article on the Honda RC 166, the small six-cylinder engine proved quite capable. Interestingly, the idea of an inline six cylinder motorcycle for production use did not quickly follow. Until Benelli decided to build the Sei.
Benelli is the oldest motorcycle maker in Europe, established in 1911. Initially it was just a small repair garage for bicycles and motorcycles. But they had the ability to manufacture all the parts they need it themselves. One of the six Benelli sons, Tonino, was a naturally gifted motorcycle racer, winning for Italian motorcycling championships in five years. As the company evolved onto building full motorcycles, his success gave serious credibility to the Benelli name.
In the earlier years, the company was very performance oriented. They developed a supercharged version of a 250 cc racing bike to compete in the 1940 Isle of Man TT 250 cc class. Pretty clever.
As the influx of Japanese manufacturers impacted the European industry, the Benelli product went down market. They even started selling small motorcycles through Montgomery Ward. Italian motorcycles in general were largely seen as anachronistic versus their Japanese counterparts.
In 1971, Argentinian Alejandro de Tomaso purchased the Benelli company, as well as Moto Guzzi. He immediately set to work with developing new models. The cornerstone of the new products was the Benelli 750 Sei.
Released in late 1972 as a 1973 model, the Sei was the first production motorcycle with a six cylinder engine. Wanting a very reliable engine, and to minimize development time, the engine was a very literal interpretation of the four cylinder Honda CB500, but with two additional cylinders. The air-cooled engine had three Dell'Orto VHB 24 mm carburetors, and six mufflers. The new motor produced 76 hp at 9000 RPM, and had a top speed of 120 mph - Not the most powerful, or fastest, but very solid performance numbers, especially for 1973.
While not a lightweight at 450 pounds, it was not as heavy as it may have looked. And most motorcycle magazines of the day who reviewed the bike were impressed with its agile handling, provided by Marzocchi forks and adjustable rear shocks, either from Marzocchi or Sebac.
The design firm Ghia was hired to style the bike, ensuring its appearance would look appropriate for years to come. The angular lines of the Sei were very representative of the aesthetics of the time. From it's debut until 1979, they sold 3200 750 Sei models.
By 1978, Honda had developed its own production six cylinder motor cycle, the CBX. Kawasaki introduced the KZ1300
in 1979. Benelli responded, introducing the 900 Sei in 1979, which was, as the name implies, the engine enlarged to 900 cc. Unlike the 750 model, it featured six into two exhaust, and a small bikini fairing. Less than 2000 were sold.
Despite its distinctive Italian character, it's performance was less on par as time passed. Since the motor was derived so literally from the Honda, it's only novelty was its number of cylinders, and even that was replicated by the Japanese. Over the years, sales fell off. Potential buyers were concerned about reliability and ownership costs, especially considering the rather steep purchase price of nearly $4000.
While there may be several six cylinder motorcycle options available today, none of them has the character of the original first production version.