03 July 2020

World's First 250cc Four - which bike?

ASK any race fan which make first went to four cylinders for a 250 and he'll probably answer, Honda. Such was the tremendous impact of the stranglehold those high-revving Japanese fours put on the world 250cc championship in the early 1960s.

But the answer's wrong. Benelli was first with a 250 racing four. What's more, it was supercharged and water-cooled. The reason it was not better known is that classic racing had already been halted by the Second World War when the beautiful little Benelli was unveiled at the Milan Show in the winter of 1939.

At that time, Italy had yet to be drawn into the war; and Benelli, along with Gilera, Bianchi and Moto Guzzi, were proudly showing the racers they had developed for 1940.

The Benelli four shocked fans and technicians alike. Only a few months earlier, Ted Mellors had convincingly won the Lightweight TT on the 30 bhp, DOHC Benelli single - the only unblown four-stroke in the 250cc class that could hold a candle to the supercharged DKW two-strokes. And experiments with a blower on the Benelli single, though incomplete, had already pushed the power up to 45hp to 8,600 to 9,000 rpm and top speed from 110 to 125 mph (unfaired).

So the blown four with 52 bhp at 10.000 rpm and a top whack of 130 mph seemed not only bold and brilliant but unnecessarily premature too.

Technically, such small cylinders were heresy in those days, for outside Italy it was widely held that 250cc was the optimum cylinder capacity. Not for another quarter-century were Honda to show that, despite the law of diminishing returns, miniaturisation of cylinders was still paying off down to 25cc.

Alas, the Benelli suffered the same fate as the blown 500 cc Velocette Roarer, insofar as its potential dominance in its class was thwarted, first by the war, then by the subsequent ban on supercharging.

Save for the power unit, the Benelli four was identical with the singles (blown and unblown). The front fork was of girder pattern, the friction-damped rear springing an amalgam of pivoted fork and plungers, and the horseshoe shape oil tank wrapped around the front of the rear wheel.

With its long tiny-bore exhaust pipes, the engine was a gem of neatness and compactness. The 42 x 45mm steel-sleeved, light-alloy cylinder block was set across the frame and inclined forward 15 degrees from vertical. The head had 24 mm valves, set at 90 deg to one another and 8mm central plugs.

On the right, a train of gears drove the two overhead camshafts. The water and oil pumps were bolted to the outside of the timing case and a half-speed drive was taken forward to the Scintilla Vertex magneto. The small radiator was mounted on the front down tube.

On the left, the drive for the large, eccentric-vane supercharger above the gearbox came off the transmission Fed by a carburettor installed on the right, the blower ran at half engine speed and delivered the mixture to a heavily ribbed, cylindrical intercooler, flexibly coupled to the four induction stubs.

With such an exotic racer built by a factory of no great size, is it any wonder that fans with long enough memories deplore the present FIM trend to restrict the number of cylinders and gears for the world championships?

Photos by Roderick Eime (c) 2018. Bike shown is located at the Peterborough Motorcycle Museum, SA.

- text from HISTORY MAKER, Motor Cycle Magazine, by Vic Willoughby, 24 February 1971

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