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BMW Pantah 500/600

(by Ian Falloon, Mar 2022)

The start of an engine dynasty

The story goes that in 1976, when the management of Ducati finally realised that the parallel twins were a commercial disaster, Ing. Taglioni smiled, reached into his bottom draw, and presented them with the full technical drawings for a 500cc V twin engine.

The origins of the Pantah went back to the Armaroli 500cc Grand Prix racer of 1973, and many of the features tried on this racing engine were incorporated in the first production models of 1979. The Pantah could have gone into production much earlier, but the EFIM management was in turmoil.

During 1977-78, motorcycle production at Borgo Panigale was at an all time low, partly due to the decline in the US market, but also because of the unsatisfactory model range, and in July 1978 control of Ducati passed to another group, the VM Group. VM were heavily involved in diesel engines, so they used the Ducati plant in Bologna for their production and development of industrial diesels.

With the control of the company in Rome, industrial diesel engine production was increased, while that of motorcycles was decreased. Finally, in 1979, after a two year delay, Fabio Taglioni was allowed to put his new engine and motorcycle into production, an engine so successful that it continues to form the basis of the entire range of modern Ducatis.

While the range of production bevel-gear twins remained strongly influenced by the singles throughout their life-span, the Pantah was a combination of the past and present. The prototype 500cc Pantah had its roots very much in the 1971-73 racers. It used the same bore and stroke of 74mm and 58mm, and like the final 500 racer of 1973, used toothed belts to drive the double overhead camshafts.

When asked by Dennis Noyes of Motor Cycle magazine in 1977 why he chose to change to toothed belts over bevel-gears and shafts, Taglioni replied, “It is no more precise but it lowers mechanical noise and will cut assembly costs. Our big V-twins are expensive to build because of the materials, and they have to be built with great care because of shimming and setting up clearances. With the belt drive we get the same accuracy without the complexity”. Taglioni also went on to say that several aspects of the design of the Pantah were compromised by the necessity to use many components from the parallel twin.

While not actually built by Ducati, the 1973 500cc double overhead camshaft, eight valve racing engine pioneered quite a few features that would eventually find their way to the production engines. The engine came from another Bolognese company Armaroli, and was based on the 500cc bevel-gear crankcases. It still had the gearshift on the right side, and primary drive and dry multi-plate clutch on the left. The exposed toothed belts were driven from the crankshaft inside the primary gears, so the drive moved from the right side of the engine to the left.

Ignition was by a set of contact breakers mounted on the external reduction gear. In a move pre-empting the Paso thirteen years later, the rear cylinder head was reversed so that both Dell’Orto carburettors faced forward between the cylinders, with a rear exiting exhaust pipe. Unlike the bevel-gear 500 racing engine, the front cylinder had radial finning, similar to the Moto Guzzi racing singles of the 1950s. Power was 74bhp at 12,000 rpm, not up significantly from the bevel-gear engines. Mounted in one of the Seeley frames with triple Lockheed discs and leading axle Marzocchis, it was raced occasionally by Spaggiari in 1973 without success.

Initial development versions of the new Pantah engine still used needle roller big-end bearings, Ing. Taglioni being reluctant to accept plain bearings. However, experience with the parallel twins had some effect on his thinking. By the time the first prototype was displayed at the Milan Show of November 1977, it had full flow oil filtration, with a spin on oil filter, and a one-piece forged crankshaft.

This show model also featured a full fairing in the style of the race kit 750/900SS fairing, and Campagnolo hydroconical brakes. Belt covers on these early engines mimicked the cylinder head finning. Not much development happened throughout 1978, and for the Cologne Show in October another version was exhibited, now with polished aluminium cam belt covers.

One month later, at Milan, the Pantah had the Speedlines replaced by six spoke FPS wheels. Production was announced to begin in March the following year, but it would several months after that before the first series was built. The pre-production bikes were painted two-tone blue and white, but the first series of 250 bikes, when they finally appeared after the summer break of 1979, were red and silver.

The Pantah deviated considerably in design from the 750/860 bevel-gear twins. It was intended for desmodromic valves only, and was much more compact. While using the 90º twin cylinder layout with vertically split crankcases, the swing-arm was also pivoted on bearings within the gearbox casing. This was done to bring the pivot as close as possible to the countershaft sprocket, reducing chain snatch. In many ways the Pantah was a mirror image of the bigger twin and earlier 500 racers.

The crankshaft was still supported by axial thrust ball main bearings, Taglioni being wary of the plain main bearings as fitted to the parallel twin, with the helical primary drive gears on the right side, and the alternator on the left. Whereas the cylinders were offset with the horizontal cylinder to the left on the 750/860, in the interests of keeping the exhaust pipes more compact, this was also reversed on the Pantah.

Also, unlike the big twin, the flywheel with ignition trigger sat inside the 200W alternator. The Bosch BTZ ignition system was new, and was adapted from the same system used on the Darmah SD900. As it was designed to be electric start only, the starter motor was neatly fitted under the front cylinder and drove through reduction gearing to a sprag clutch screwed to the back of the flywheel.

The toothed belts and valve gear were driven off a jackshaft running between the cylinders and geared from the crankshaft on the left side. This enabled the engine to kept much narrower (at only 14.8 inches or 776mm) than the bigger engines that needed to fit the geared camshaft drive inside the alternator rotor.

The primary drive, with a ratio of 31/69 or 2.226:1, drove a standard Ducati style wet multi-plate clutch, something has been under continued refinement by the factory. It is beyond the scope of this book to detail every modification to Ducati clutches over the years, but generally they occur at almost annual intervals.

If there has been an Achilles heel to the Ducati engines over the years, it has been the clutch, and as the engines get successively more powerful, the clutch slipping problem is exacerbated. The Pantah clutch too was the reverse of the 860, in that the six springs clamped the driving and driven plates from the inside outward of the alloy clutch drum and hub.

The forged one-piece crankshaft necessitated the change to two-piece connecting rods with plain bearing big-ends, and a corresponding increase in oil pressure. Whereas the old engines, with all their ball and roller bearings, could run with only 15psi from its geared pump, the Pantah flow rate was regulated to 70psi. The geared oil pump resided in the same location as the earlier engines, but was now driven by the helical primary drive gear.

Unlike the larger twin, maintaining a short wheelbase wasn’t such a problem, so the gearbox was the indirect type, with separate input and output shafts. Consequently the engine rotated forward, not backward as the 750/860. As with the 860 though, the first engines (to engine number 3245), used gearboxes with six engagement dogs. Following racing 860 practice, these became the stronger three dog type in 1982.

Much of the reason for making the Pantah a mirror image of the earlier twin was so that the gearshift could be properly incorporated on the left side. The cylinder barrels were also Gilnisil (an Italian Nikasil), a plating incorporating silicon and carbon particles that was much harder than iron, lighter, harder wearing, and offered better heat transfer. The only down side was that the cylinders couldn’t be rebored.

The two valve cylinder heads used the 60º included valve angle of the 1973 racing bikes and parallel twins, with 37.5mm inlet and 33.5mm exhaust valves. The desmodromic valve actuation system mirrored Taglioni’s design for the parallel twin 500 Sport, but with the different timing figures of inlet opening 50º before top dead centre and closing 80º after bottom dead centre. The exhaust opened 75º before bottom dead centre, closing 45º after top dead centre. Feeding these cylinder heads were 36mm Dell’Orto PHF carburettors, restricted by a large air filter on top of the engine, under the fuel tank. Compression ratio was 9.5:1, and claimed power was 52bhp at 9050 rpm.

Supporting this engine was a trellis type frame, also designed by Taglioni. Two pairs of parallel tubes running from the rear cylinder to the steering head met another pair of tubes running up from the rear of the crankcases. This was braced for extra rigidity.

The engine hung below the trellis and was bolted to it at six points. It wasn’t the most compact frame as the wheelbase was still 57 inches (1450mm), but it was considerably shorter than the larger twins. Suspension was by Marzocchi front and rear, with 35mm diameter forks and 310mm shock absorbers. With clip-on handlebars, and a half fairing, the 180kg (396lb) Pantah 500SL was still very much a sporting motorcycle. The Nippon Denso instruments and the switchgear were the same as on the Darmah.

When the first production models finally appeared during 1980, they were painted a pale blue, with red and dark blue stripes. The styling didn’t meet with universal acclaim, and the first 500SL’s were unlike other V-twin Ducatis in that they had very little bottom end and mid range power.

As delivered, the Pantah was an extremely quiet bike with its toothed belt camshaft drive, rubber plugs in between the cylinder fins and quiet Contis. The engine liked to rev, but unfortunately the gearing was so high at 2.533:1 that performance was limited, and the bike wouldn’t run near to its power peak in top gear. Motor Cycle Weekly magazine managed 114.34mph (184km/h) in October 1980 running to only 8500rpm in top gear.

Contemporary tests were extremely complementary about the handling of the Pantah. With a 30.5º steering-head angle, the Pantah was a stable motorcycle, and the frame more than sufficient for the modest weight and power of the 500cc engine. My experience with 500 Pantahs was that they handled well enough, and steered slowly and predictably, yet did exhibit signs of looseness somewhere between the rear wheel and steering head over bumpy roads. The 35mm forks were lacking in rigidity too, especially by modern standards.

The engines though, while lacking in torque compared to an 860, were thoroughly reliable and oil tight. A small practical problem with Pantahs though was the slow discharge of the 12V 14Ah battery if they weren’t used regularly.

In line with past Ducati practice, it was obvious that a simple overbore would be the first step in the development of the Pantah. Already, early in 1980, a 600 Pantah with a Mike Hailwood Replica style full fairing, had been seen at the factory, and a factory racing kit that included larger pistons was marketed for the 500, just as the first production bikes became available.

The new 600SL was displayed late in 1980 along with a prototype turbocharged Pantah engine. Nothing came of the turbocharged version, but the 600SL became available in early 1981, with silver paintwork, a new fairing and a hydraulically actuated wet clutch with stronger springs. Paioli 35mm forks were fitted on the earliest versions, and throughout 1981 and 1982, either Marzocchi or Paioli forks and shock absorbers appeared on the production bikes.

These also had larger, 08 Brembo front brake calipers, mounted behind the fork legs, and a larger, 400H18 rear tyre. The 600SL was unchanged throughout 1982 but for a black plastic front guard. Claimed dry weight was up to 187kg (411lb).

The 500 Pantah continued alongside the 600, and in 1981 received the 600 style fairing, but not the hydraulic clutch. The engine cases were standardised as they had been with the square-case 750/900SS in 1975, so that in effect, from number 1654 the 500 was a sleeved down 600 (for some reason 290 engines after this had the earlier cases).

The extra capacity for the 600 was gained in exactly the same way as the 750 had become an 860, by a 6mm cylinder bore increase. With an 80mm bore and the 58mm stroke, capacity was 583cc, with compression still 9.5:1. Valve sizes, valve timing, and carburettors were shared by both models.

Claimed power was 58bhp at 8500, but by 1982 it had risen to 61bhp at 9100rpm with compression now 10.4:1. For the 500 the power went down to 45bhp in 1982, with no change in specification. There were many aberrations between claimed power and weight figures for various models at this time, but it is difficult to believe that a simple 17% capacity increase would equate to a 35% power boost.

The 600SL was even more highly geared than the 500 at 2.4:1, and also wouldn’t run to the red-line in top gear. The English Motor Cycle News in April 1982 could only manage 117.29mph (188.8km/h), in fourth gear. It was five mph slower in top!

Both the 500 and 600SL Pantahs were listed through to 1984, by which time the 600 was painted in the Mike Hailwood Replica colours of red, with green and white stripes, with a red frame. These last 600s had also reverted to a cable operated clutch and now had a larger, steel clutch basket. The last 500SLs were white with red stripes.

For the Italian market, a 350SL appeared in 1983, also in the MHR colours, but with silver FPS wheels. The 349cc engine, with a 66 x 51mm bore and stroke, 10.3:1 compression, and 30mm Dell’Orto carburettors, produced 40bhp at 9600 rpm.