DUCATI 750 GT
Article #3, 8 April 2011 by Hans Smid
In the early seventies Fabio Taglioni was at the end of technical possibilities with the single cylinder 450 desmo. The successful engine could not be enlarged anymore without losing the properties it was so beloved for.
Ducati Meccanica was in desperate need for something completely new in the race for bigger, faster and more, which had just begun after Honda's daring project with the successful CB750. One was not taken seriously anymore with a bike under 650cc...
Taglioni developed a technical masterpiece in 1964 known as the 1200 Apollo, developed for the American Police force. Regretfully it didn't come any further than the stadium of prototype, as this gigantic motorcycle was decades ahead of its time and too powerful for the available tires. Under pressure of Ducati managers Fredmano Spairani and Arnoldo Milvio orders were given to Taglioni to design a new revolutionary engine concept that would lead Ducati into the heavier class. The result was both simple and brilliant.
Fabio combined two bevel drive single cylinders on a common crank-shaft that led to the first, nowadays so characteristic, Ducati trademark: the L-twin. When in spring 1970 the drawing tables were cleared for other designs and ideas, the first prototype was tested in July. And with success, because only sixty days later the first Ducati 750 GT struck the Italian press like thunder. Though sporty looking, the driving characteristics were more those of a touring model.
Because of the L-twin concept, this Ducati was "built around", it became a model with a rather long wheelbase (1530 mm). The engine was equipped with 28 bearings, no less than 9 of those just to lead the bevel gears, so the idea might have been simple, the construction wasn't. The 750 GT by the way was the first Ducati with 12 Volt electric circuit. The model was surprisingly light with 185 kg. The road holding was quite excellent and proved a good starting-point for developing a more sporting machine, the later 750 Sport.
The first Ducati 750 GT’s of 1971 and 1972 were fitted with Amal concentric carburetors and a ‘leading axle’ Marzocchi front fork with polished legs and Lockheed twin piston brake callipers on a single disc, later replaced by Scarab brakes in 1972. Early models also featured Smiths instruments until 1973 when they were replaced by Veglia instruments.
1973 models were also fitted with Dell’Orto PHF30A carburetors and had a steel fuel tank and painted steel mudguards, instead of the stainless steel as used before. Further changes occurred in 1974 when the engine design got a mild update and a centre axle Ceriani fork and a Brembo brake disc was fitted.
Taglioni’s brilliant engine design had a combination of bevel gears, used to drive the two camshafts. It is extremely time consuming and takes a high level of skill to assemble these with the correct backlash on all gears. It took up to one day of production for a skilled worker to manufacture one single engine. It became clear that an engine redesign was necessary to cope with the financial department and government regulations.
Production of the 750 GT seized in 1974 and with it came an end to one of the best motorcycles ever produced in Borgo Panigale. It marked the end of the ‘round case’ 750 L-twin era, a common name given to this engine type to distinguish it from the later ‘square case’ design, first introduced on the 860 GT in the same year 1974. For many enthusiasts, the handling, the drive, the comfort and the excitement the Ducati 750 GT has to offer is unparalleled with any other bike in history.
The 750 GT ‘round case’ engine has vertically split aluminium crankcases, a 90 degree L-twin layout with the vertical cylinder inclined at 15 degrees upwards. The five-speed gearbox is incorporated in the engine crankcase and all 750 GT’s have a right side gearshift. The valves are actuated by single overhead camshafts driven by a set of bevel gears and uses valve springs for closing (non-desmo). Early 750 GT’s (until 1973) were fitted with Amal concentric carburetors, later replaced by Dell’Orto PHF’s. Some 750 GT’s of 1973 and 1974 feature a Marelli starter motor.
The 750 GT frame was a completely new development, although it incorporated some of the basics of the earlier ‘wide case’ single frame. With its tubular steel frame, that uses the engine as a stressed member, the 750 GT soon earned a reputation to be one of the best handling bikes available. Early models were fitted with a leading axle Marzocchi front fork with polished legs and a Lockheed disc brake to be replaced in 1973 by the same front fork with black painted legs and a Scarab disc brake. The 1974 750 GT is fitted with a centre axle Ceriani front fork and a Brembo disc brake.
References (recommended further reading)
Falloon, Ian (2004) ‘ Standard Catalog of Ducati Motorcycles’, Iola: KP Books
Falloon, Ian (2006) ‘The Ducati 750 Bible’, Dorchester; Veloce Publishing
Falloon, Ian (1998) ‘Ducati Twins Restoration Guide, Bevel Drive 1971 – 1985’, Osceola; MBI Publishing Company