In its various evolutions the 16-valve air-cooled four followed the same basic format as that first Pagani 350 in 1971, over square, lots of revs and resulting in squat, square-finned cylinders inclined forward by 10 degrees. From delivering 69bhp at the gearbox at 14,000rpm in original 52 x 40.4mm guise, the stroke was progressively shortened, first to 53 x 39.5mm, in which form it won the second of its world titles in 1973, by then delivering 70bhp at 16,000rpm. For its 1976 return, Mazza designed an ultra-short stroke version measuring first 53 x 38.2mm, then in its final evolutionary guise, an amazing 54 x 38mm. In this form the MV delivered 77bhp at 16,400rpm, and as that Mugello carelessness with the gearing showed, would survive being revved sky-high without exploding!
The engine's six-bearing crankshaft, with small flywheels for minimal inertia, incorporating integral crankpins, was bolted into the compact, heavily machined, rough-cast crankcases with a ball bearing on one side and roller on the other, with MV's trademark long, finned sump containing three litres of oil slung underneath - a separate oil tank for a dry-sump motor.
However, though nominally air-cooled it was essentially oil-cooled, with minimal finning, a high-capacity oil pump, large-diameter oil hoses and a big oil radiator in the fairing nose. As a further mark of their forward thinking, the side-loading six-speed gearbox was extractable as per modern practice, but was unheard of on Japanese two-strokes, which would have benefited even more from being able to change internal gear ratios quickly and easily.
As the MV's revs reached for the sky and its cam profiles became more aggressive, it needed an ideal gearbox for each circuit, from five different ratios available for each gear. The Swiss-cheese rear sprocket had a rubber cush-drive incorporated in the hub of the US-made Morris cast magalloy wheels, which replaced the Borrani wire rims used previously. That same year, 10in (254mm) Hunt plasma-sprayed alloy brake discs, also made in the USA, were fitted replacing the Italian Scarab iron front discs, in the interests of reducing unsprung weight and lightening the steering.
Gear drive to the twin overhead camshafts lay up the centre of the engine, with the four very long valves (about 120mm in length, to avoid the springs masking the ports with such a narrow valve angle) per cylinder sitting at an angle of just 35 degrees: compression ratio was 12.2:1 from the two-ring, ultra-slipper pistons and the flat combustion chamber. The four DellOrto carbs had a steep downdraught - and the exhaust pipes quite a steep updraught! Carb sizes varied greatly from one circuit to another, in an effort to change power characteristics, with choke sizes ranging from 28mm up to 34mm, and each pair of carbs sharing a remote float chamber, to save weight and width.
Ignition was the bike's achilles heel in short stroke form, where the bevel gear-driven Mercury outboard engine magneto used successfully until
1974 could no longer spark the single central 10mm plugs. After various experiments, two Krober electronic ignition boxes were fitted in its final Brands Hatch guise: at least it didn't hole a piston in that race
MV used five different chassis designs for their 70s fours: the final 350 version was an open-cradle duplex with large-diameter chrome-moly steel tubes employing the engine as a semi-stressed component, whose most notable feature was the eccentric mount for the swinging arm pivot, which allowed the team to experiment with droop in an effort to improve the handling and traction of the twinshock rear end.
Though the bike was only raced on slicks a couple of times on fast circuits, they supplied enough grip to cause handling problems with the 500, though not so much with the smaller: Agostini was noted as being the only rider who could take the famous double kink in the straight, behind the pits, flat out aboard the MV, without backing off, on his ride to victory in the 1976 Dutch GP. With a tiny 1280mm wheelbase (the same as the 2002 Aprilia 125GP racer!), the MV was quick-steering and nimble, and in spite of its increased complexity, contrary to popular belief, it didn't lose out on weight against its two-stroke rivals, scaling 121kg dry against a TZ350's 118kg - before adding water! Once the Ignition problems were cured, it was an even match.
Above: State of the art four stroke in 1976 but the heritage is still obvious.
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