The 750 Sei was presented at the end of 1972, as a prestige project of Alejandro De Tomaso, who had taken over Benelli in 1971. The engine was by Piero Prampolini, the styling by Paolo Martin (Ghia). It took almost two years before the bike was really on sale in 1974.

It is often said that the engine is a Honda CB500 Four with two extra cylinders, with the alternator moved behind the cylinders to keep the engine narrow. The English Wikipedia tells it that way; Motorcycle Classics calls that disputed, but confirms that bore and stroke are the same as the Honda’s.

The Sei was the first six-cylinder motorcycle in series production and remained so only until the 1978 Honda CBX. According to the Italian Wikipedia it was also Brembo’s first motorcycle project with disc brakes. According to several sources about 3,200 were built in total. Sources differ on the split per year; Motorcycle Classics gives 293, 1,479, 1,145 and 283 for 1974 to 1977.

The Sei has no racing history: it was a luxury touring bike and a flagship for the make.

750 Sei (1974–1977, according to some sources until 1978): three twin-choke carburettors, six separate exhausts (‘six-into-six’).

900 Sei (from 1979; Classic Bike Hub says 1980): 906 cc (60 × 53.4 mm), approx. 80 hp at 8400 rpm, with a small fairing and a six-into-two exhaust. About 1,800 to 2,000 were built, until the mid-eighties.

On this page we mention the 900 Sei only for comparison; the points to check are similar.

The engine is less robust than a Honda’s. Oil leaks and oil consumption are known complaints. The valves quickly go out of adjustment and the cam chain needs regular tensioning.

From the factory it has contact-point ignition. According to Classic Bike Hub, conversion to electronic ignition is practically essential for reliable riding.

Early gearboxes sometimes broke under hard acceleration. That was solved around 1975; a Sei from 1975 onwards is therefore preferable.

Poor connectors and switches, breaking speedometer and rev counter cables and leaking fuel taps and caps are common. On the 900 Sei leaking front forks, a slipping clutch and an expensive double (duplex) drive chain are also mentioned.

Brakes, instruments and switches were shared with Moto Guzzi of the same era and are reasonably easy to find. Upholstery, trim and especially the six exhausts are scarce. Classic Bike Hub advises buying a good bike rather than a project, and joining a club. The Spanish Benelli club has a parts list for the Sei 750 and 900.

Price indication: a 1978 750 Sei sold at Bonhams (Stafford) in April 2024 for 22,425 pounds. A 900 Sei fetched about 10,350 pounds in June 2024. For a 750 Sei, expect roughly €10,000 to €26,000 (2024), with top prices for intact, original examples.