Benelli 250 Single Cylinder

Benelli 250  1938

The Benelli 250 made its debut on the racing scene in 1934, after the abolition of the Italian speed championship for light motorcycles with up to 175-cc. displacement. The Benelli 250 was certainly the most glorious racing motorcycle built by that company. The vehicle made its appearance at a lucky moment, on the crest of the winning streak of the Benelli 175 single- and double-shaft. The 250 had the same basic features as the 175.

The Benelli 250 had the task of continuing in a new class the winning streak that had been started by the 175. This was certainly one of the reasons that the company did not try for any technical innovations on its vehicles. The technicians based all their work on past experience. The new engine was a single-cylinder two-shaft with geared distribution, dry-sump lubrication, and separate transmission. The main difference was that because of the increased displacement, a long-stroke solution was used and the single exhaust valve communicated with two tubes rather than one.

Before it went into racing competition, the Benelli 250 set a world record in its class for the flying kilometer, 181.818 km./hr. (about 113 m.p.h.)—an outstanding achievement considering that this speed was faster than the record set by a 350.
The Benelli 250 did not outperform the finest Guzzis, however, until 1936. When the 250 started to show its stuff, the Benelli people decided to modify the engine again. A different bore and stroke were introduced.

The new Benelli 250 single-cylinder Benelli 175 Two-shaft. The 250 was developed from this model
won some important victories in 1938. The following year the British racer Ted Mellors became its rider. He had driven an official Velocette against the Benelli in the past. Now Mellors won the 250 its first Tourist Trophy. Even more important, the motorcycle beat the official Guzzi and DkW teams with their superchargers.
In 1940 the Benelli 250 got a supercharger as well. This new engine should have generated 35 h.p., but it was never put to the test because of the outbreak of World War II.

The Moto Benelli company was bombed and plundered during the war and many of its racing motorcycles ended up in unknown hands. But some of the other machines had been hidden away, and these were among the first motorcycles to go back into racing in the postwar years.

Benelli won races again in 1948 with old-fashioned motorcycles (the 250 two-shaft version without supercharger), thanks in part to the skill of Dario Ambrosini, a fine racer who had formerly driven for Guzzi. In 1950 the old single-cylinder was still technically superlative and Ambrosini raced it with great skill, winning the 250-class world championship.

Motorcycle: Benelli 250 Single-cylinder Manufacturer: Moto Benelli, Pesaro Type: Racing Year: 1938
Engine: Benelli single-cylinder, four-stroke, with two-shaft overhead geared distribution. Displacement 248.8 cc. (65 mm. x 75 mm.)
Cooling: Air
Transmission: Four-speed separate
Power: 27 h.p. at 9,500 r.p.m.
Maximum speed: 112 m.p.h.
Chassis: Tubular, open, single cradle, with stamped-plate parts. Front, parallelogram suspension; rear, elastic hub suspension
Brakes: Front and rear, side drum

Benelli 250 1939

In 1939 Benelli machines appeared in the Isle of Man for the first time; two were entered for the Lightweight T.T. One retired—and the other, ridden by Ted Mellors, won by almost four minutes.
The machine that scored this success is interesting because its conception and general pattern established a style that was to become commonplace after the war in Italian 250 and 125 racers. Light alloy was used very extensively, for barrel and head and for a bolted-on cam-box. A gear train drove twin overhead camshafts completely encased but operating valves with exposed hairpin-type springs.

The magneto drive was by an extension of the gear train forward of the crankcase. Astern a four-speed gearbox was bolted up to form a unit. Another feature, pioneered by Guzzi, of Italian lightweight-styling was the use of an outside flywheel. The machine had a deeply finned crankcase, a large oil tank and an oil cooler, in an endeavour to promote cool running; all this proved a bit unnecessary in the 1939 race, for it was exceedingly wet—so wet in fact that for the race a hastily improvised cover was fitted over the magneto.

The rain slowed the race speed to a mere 7425 m.p.h. Graham Walker rode the little model just after the race and reported it to be "unburstable". No rev-counter was fined and Ted Mellors told him to rev it "until the exhaust note doesn't alter". This was about 8,200 r.p.m. on the level in top or 9,000 in the gears—a top speed of about 110 m.p.h. which was still good for a "250" twenty years later.

SPECIFICATION
Engine: single-cylinder 250 c.c. d.o.h.c; light-alloy
cylinder and head; gear drive to camshafts. Ignition: magneto.
Transmission: chain drive via four-speed gearbox. Frame: single down-tube cradle, with swinging-fork
rear suspension controlled by "plunger-type"
spring boxes. Forks: single-spring girder with friction dampers.

Description

Following the success of its 175cc single-cylinder racer and the derivative 250 version during the early and mid 1930s, the Benelli factory embarked on the creation of a new 250 racer for the 1938 season.

Devised by brothers Giuseppe and Giovanni Benelli, the new machine bore resemblance to its 250 predecessor, but it was pretty much an entirely new design. The engine was still a vertical DOHC single, but with a new 65x75mm bore and stroke, capacity was slightly increased from 246.7 to 248.8cc. The twin overhead cams were driven by a redesigned geartrain, which also drove the Marelli magneto and oil pump. The oil feed to the now fully enclosed valve gear was cast into the geartrain’s alloy cover, and the ’38 250 also featured a new, Benelli-designed 4-speed gearbox. A bigger 28mm Dell’Orto carburettor and a new long-length exhaust also contributed toward 26bhp at 8,400rpm – sufficient for a competitive 115mph top speed, although there wasn’t useful power below 6,000rpm.

The simple cradle frame and front girder forks remained relatively unsophisticated, but the new 250 did boast swinging arm rear suspension – a significant step forward from the previous model’s rigid rear end.

The 1938 European Championship season went well for Benelli. Although DKW eventually took the 250 title, the Pesaro firm’s machines took all three podium positions in the Italian 250 GP at Monza, a race run concurrently with the 350cc class. As a bonus, the first two 250 Benellis beat the 350 class winner Ted Mellors and his Velocette.

This Monza performance so impressed multiple GP winner Mellors, that he approached Benelli about securing a 250 for the following year’s Isle of Man Lightweight TT.

Briefly, Benelli lent him a 250 Bialbero, along with just one mechanic and a few spares, and this was sufficient for Mellors to be fastest in practice and secure both his and Benelli’s first TT win, beating formidable opposition from the DKW and Moto Guzzi factory teams and their supercharged machines.

Source coys.co.uk