1968 Ducati 250 Mark 3 Desmo
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Ducati is so associated with V-twins these days that it’s hard to remember that it was its bevel-drive overhead cam singles that were the proving ground for Fabio Taglioni’s famous desmodronic valvetrain. Any of these sporting 250cc, 350cc or 450cc singles from the 1960s are noted for crisp handling and crackling exhaust, and they don’t often get the credit they deserve.
The “desmo” setup permitted a 20 percent increase in engine revolutions by eliminating valve float, which occurs when a valve spring cannot close the valve quickly enough to complete the ignition cycle. Mercedes-Benz developed the first effective system for its W196 and 300SLR racers in the early 1950s, but never put it into street cars as it was an expensive piece of technology.
Taglioni has a long career at Ducati 1954-89 and almost immediately. For 1956, he developed a twin-cam, 125 cc desmo and an example went right out and won the Swedish GP, lapping the entire field at 12,500 rpm. A Grand Prix model was sold for the street until 1962 and remains a very rare sight today.
Over the following five years, Ducati sold a dizzying range of engine sizes and style, and this included two-strokes. The inclined cylinder, alloy case overhead cam single, though, was in production for almost 20 years and grew from 125 to 175 cc, then to 200 cc, and then 250/350/450 cc units. Road-going bikes used spring-valves from 1957-67 before the desmodromic valves were finally sold for the street.
The first sporting 250 cc Diana appeared in 1961, along with a touring version called the Monza and a Motocross version called the 250SCR in 1962. These followed the “narrow-case” design of 1957, in which the front and rear engine mounts are the same width. The Diana was a competent performer. It weighed just 265 lbs and could do 85 mph.
The Diana Mk 3 Super Sport came in 1963 with clip-on handlebars, racing guards and tires and a 5-speed gearbox. What really sold the Ducati name in the US, though, was the 250 Mach 1 of 1964, a bike that made 27 hp and could do 106 mph on a wildly optimistic 150 mph speedometer. Though the Mach 1 was discontinued in 1967, its highpoint was probably 1969, when Alastair Rogers won the 250 cc TT race in the Isle of Man at an average speed of 84.79 mph.
When shopping, it’s not a bad idea to avoid numerous less desirable touring and dirt bike configurations, including the Mountaineer 100, Cadet 100, Bronco 125, Monza 160/250, the 250GT, 250 SCR and 350 Sebring. All have underwhelming performance, dubious reliability, fragile kick-starts and unreliable electrics.
In 1968 the “wide case” 250 cc and 350 cc engines were introduced and remained in production until 1974. Engine mounts were set further apart, which stiffened up the frame, allowed for a larger sump and enabled stronger kick-start gears to be used.
Amid intense racing activity, the new single-cam desmo engine was made available and once you had pitched the strangling Silentium muffler, 99 mph was attainable on the 250 cc Mark 3 and 112 mph on the 350 cc Mark 3 version. Desmos are recognizable by the “D” on the side panels.
Ducati singles were made through 1974, with sprung valve engines sold alongside desmos. A 450 cc engine was introduced in 1969 and all models received bigger Marzocchi forks, Grimeca double-sided front brakes and Borrani alloy rims in 1971. That year saw the “Silver Shotgun”, so-called for its striking metal flake paint.
With a dizzying mix of models available through this period, it’s worth noting that the best buys are the narrow case 250 cc Diana MK III, 250 cc Mach 1, and wide-case 350 cc Mk III Desmo. Bikes with few leaks are best, as are the ones with the quietest running motors with easy starting and reliable idling.
Weak points are the narrow-case kick start mechanism and flywheel magneto ignition. Hard-to-find parts include Mk III megaphone, clip-ons, rear sets, Diana/Mach1 fuel tanks, jellymold fuel tanks, stock tire pumps, Veglia mechanical tachometers, factory flyscreens, tach mounts and tach drives.