The Ducati parallel twins are a series of 350 cc and 500 cc parallel-twin SOHC motorcycles produced by Ducati from 1975 to 1981. They were also built under licence in Spain by MotoTrans until 1983. Sales were poor and the machines were unreliable, earning them the nickname of paralysed twins.
Ducati 500 at a glance
| Maker | Ducati, Borgo Panigale; also MotoTrans, Spain |
|---|---|
| Production | 1975–1981; Spanish production to 1983 |
| Type | parallel twin, single overhead camshaft |
| Crankshaft | 180°, to reduce vibration at high speed |
| Cylinders | vertically split crankcases, cylinders canted forward 10° |
| Carburettors | Dell’Orto 30 mm on the 500, 26 mm on the 350 |
| Gearbox | five-speed, left-hand change |
| 500 Sports Desmo power | 50 bhp at 8,500 rpm |
| 350 Sports Desmo power | 42 bhp at 8,500 rpm |
| Models | 500 GTL, 500 Sports Desmo, 350 Sports Desmo, GTV |
History of the Ducati 500
Why a parallel twin
In the early 1970s Ducati was in financial trouble and owned by the Italian government. In 1972 the engineer Cristiano de Eccher became general manager and decided the way to turn the company around was to cut production costs and sell more machines. Ducati’s reputation had been built on the OHC single, by then outdated, only moderately powerful and expensive to make. The Japanese were selling many parallel twins, and Ducati management believed it could take a share of that market with a similar twin.
The rejected V-twin
After the success of the 750 GT V-twin, chief engineer Fabio Taglioni proposed a smaller V-twin with belt-driven camshafts to reduce costs. De Eccher rejected it and announced at a 1973 press conference that the OHC singles would stop and be replaced by a new parallel twin. Ducati already had a twin design: at the request of the American importer Berliner, Taglioni had made a 360° OHV twin shown to American dealers at Daytona in early 1965, but it was never produced because of poor performance. Taglioni declined to develop it and Bruno Tumidei took over, while Taglioni spent his time on the belt-drive V-twin that became the Pantah.
The engine
Tumidei updated the engine with ease of manufacture as the main aim. The vertically split crankcases and 10° forward-canted cylinders were kept, and, unusually for Ducati, a forged crankshaft ran in plain main and big-end bearings with no centre main bearing. A 180° crankshaft reduced vibration at high speed but increased it at low and medium speeds, and because the engine had been designed for a 360° crank with large counterweights the crankcases were bulky. The camshaft was driven by chain from the back of the clutch hub via a countershaft behind the cylinders, and the head used a 60° valve angle like the Imola racers. A remote oil reservoir inside the engine avoided an external tank but brought complicated plumbing, frequent oil changes and the need to watch oil levels. The five-speed gearbox had a left-hand change to comply with American law, and there was an electric starter. The engine suffered reliability problems, especially oil feed to the camshaft, and build quality was poor.
GTL, Sports Desmo and GTV
The GTL used a single downtube frame with the engine as a stressed member, Marzocchi forks, twin 320 mm Marzocchi shocks, battery-and-coil ignition with points and Smiths instruments, in Giorgetto Giugiaro’s angular style. The 500 had twin 260 mm discs and the 350 a single disc, with a 260 mm drum at the rear. After Franco Zaiubouri replaced De Eccher in September 1975, a more attractive model was planned: Taglioni designed a desmodromic head giving 50 bhp at 8,500 rpm on the 500, though with a narrow powerband. Leopoldo Tartarini styled it with a twin downtube frame, a 14-litre fibreglass racing tank, FPS cast wheels and a rear disc, in red and white. The 500 Sports Desmo went into production in 1976 and a black-and-yellow 350 version followed in 1977, with 42 bhp; most Sports Desmos were built at the Italjet factory with Ducati engines. The GTV of 1977 was a Desmo fitted with a GTL engine, Japanese switchgear, a Bosch headlight, touring handlebars and a chrome exhaust.
Design and features
- Cost-driven design: a parallel twin chosen over Taglioni’s proposed small V-twin to fight Japanese competition.
- 180° crankshaft: smoother at high speed, rougher at low and medium revs.
- Sports Desmo: a desmodromic head raising the 500 to 50 bhp, at the cost of a narrow powerband.
- Reputation: reliability problems, especially camshaft oil feed, and poor build quality.
Ducati 500 versions
- 500 GTL (1975–1981): Giugiaro-styled standard twin, twin front discs.
- 350 GTL: the smaller version with a single front disc.
- 500 Sports Desmo (1976–): Tartarini-styled, 50 bhp, FPS wheels and a rear disc.
- 350 Sports Desmo (1977–): the 42 bhp version in black and yellow.
- GTV (1977–): a Desmo with the GTL engine and touring equipment.
The parallel twin was meant to replace the OHC singles such as the 350 Mark 3, and its failure cleared the way for the Ducati Pantah. More Ducatis are in the Ducati collection.
Video: 1978 Ducati Desmo 500 Sport
Video by shwnmt on YouTube.
Sources
- Ducati parallel twins, English Wikipedia: background, engine, models, production
