“We’d made a 350 cc version of the 250 twin, which by now was liquid-cooled,” Fabris says. “We also built a 385 cc version of that, and in the final round of the 1973 500 cc Italian Championship at Misano, Bonera gave Phil Read on the four-cylinder MV Agusta a lot of trouble throughout the race, and only narrowly finished second to him, ahead of all the other true 500s, after setting the fastest lap. Well, that set us all thinking about what might be possible with a purpose-built full 500, and since Walter Villa, who had by then also started to ride for us, didn’t like the 385 and wanted to race a full 500 cc bike, we decided to ask our management to let us build one.”
The answer from Harley’s Italian subsidiary’s management was a positive one – but with certain conditions. Though its Italian spinoff was doing okay, the U.S. parent company, then controlled by AMF, was suffering a slide in sales owing to quality issues and increased Japanese competition, so there was a strictly limited budget to develop the bike, and it would have to be offset by making a customer version available. A minimum of 25 examples of this had to be built in order to homologate it for AMA competition. Otherwise, go ahead, guys – see what you can do!
As always in Italy, passion overcame whatever restrictions might have been thought to be imposed, and during the spring of 1974, engine designer Egisto Cataldi – already the creator of the 250/350 cc parallel-twins that would shortly gather up a total of four World titles – worked after hours at home to design the RR500 engine from a clean sheet, with only the gearbox casing and some minor details carried over from the smaller engines.
Overcoming Limitations
Riding The Harley Davidson RR500Like the smaller powerplants, the new Harley RR500 was also a liquid-cooled parallel-twin with a 180-degree crank, and its six transfer/dual intake/single exhaust port cylinders that were canted forward 15 degrees from vertical measured 72 x 60 mm for a capacity of 488 cc.
Unlike its piston-port predecessors, the new Biancone (as in “Big White,” referring to the fact that the factory bike raced in a plain white fairing for a couple of its early races) had a reed-valve engine, in an attempt to soften the power delivery and make the bike more tractable on tighter tracks. However, the problem was that there wasn’t yet a reed valve pack big enough for the bike’s requirements – nobody made them at all in Italy, so they couldn’t even persuade someone local to make them a special design.
The only solution was to use a pair of Yamaha reed valves on each cylinder sourced from the newly launched four-cylinder TZ750, so that meant doubling up on carburetors, too, with two 34 mm Dell’Ortos mounted to each cylinder with the reed blocks splayed apart at an angle for clearance. These were later replaced on the single factory racebike by 34 mm Mikunis, while the total of 18 RR500 engines built (not 25, whatever they told the AMA!) resulted in 16 complete motorcycles being sold to customers that winter for 3,500,000 lire, all of which retained the Dell’Ortos.
Ignition was provided by a self-generating Dansi CDI mounted on the left end of the crank with just 21 degrees of advance, and the 89 hp delivered to the rear wheel at 10,000 rpm (with maximum torque of 50 ft-lb at just 6,800 rpm – quite a two-stroke tractor!) was transmitted via an extractable cassette-type six-speed gearbox with different standard ratios from the 250/350, matched to a 16-plate Surflex dry clutch positioned on the left behind the generator.
Bookmarks