The New Harley-Davidson (aermacchi) Road Racers
THE NEW HARLEY-DAVIDSON (AERMACCHI) ROAD RACERS
FEATURES
Watch Out, Yamaha, A New 250/350 Machine Is On Its Way To AMA Privateers
CARLO PER ELLI
This H-D Aermacchi is neither four-stroke nor Single. And it is potentially a great weapon for the AMA 250 class, judging by Pasolini’s excellent ride against the factory Yamahas at the Italian Grand Prix. These new racers originated from the Italian Aermacchi 125 Single, and are, in effect, a result of coupling the Singles to make a Twin . . .
THE 250 & 350 Aermacchi/H-D two-stroke Twins, which performed so well at the 50th Italian Grand Prix at Imola with Pasolini in the saddle, will be offered for sale in limited quantities. The Varese factory is famous for its support of the private rider and its successful over the counter 250,350 and 420cc pushrod Singles (now discontinued) and the 125cc two-stroke Single, the Golden Wing, have all done well in competition.
These new racers originated from the 125cc Golden Wing by coupling two engines together to create the first 250, and then later overborflig it for the 350. First designed near the middle of the 1970 season in Italy, and campaigned by Pasolini at Modena early in 1971, the new Twins started to get some decent placings near the end of the season. Initial bugs with the ignition and gearbox were quickly sorted out.
But even these improvements were not satisfying to Aermacchi’s chief twostroke engineer, William Soncini, and
during the winter the machines were redesigned from stem to stern.
Salient design features are horizontally-split crankcase assembly and a separate gearcase assembly which permit removing either half of the two-piece crankshaft (which is held together with a sleeve and half-moon keys) or the gearbox without distrubing the other assembly!
The six-speed gear cluster can be inspected through a cover in the top of the gearcase, and the entire cluster can be easily removed from the case from the left-hand side. The dry-type clutch is exposed on the opposite side and nearby a transparent plastic window enables checking the gearbox oil level at a glance.
The various transmission gears, running on caged roller bearings, are all independent from the layshaft and mainshaft so that each one can easily be changed for repair or to change the internal gear ratios.
A 5 percent petrol/oil mixture without the use of an oil pump is employed and an electronic ignition system, developed by Dansi, provides the sparks with utter reliability. In order to produce a competitive, relatively low-cost machine, Aermacchi/H-D technicians haven’t tried to install water cooling or rotating disc valves, but they realize their advantages.
For optimum “squish” effect the cylinder heads’ combustion chambers are jocky cap shaped, and radial finning has been chosen for optimum cooling. A power figure of “over 50 bhp” for the 250 and “over 60 bhp” for the 350 is claimed with the 250’s power band being from 9800 to 11,400 rpm and the 350’s being from 7,800 to 10,800 rpm.
The light alloy chromed bore cylinders have four transfer ports in addition to the usual inlet and exhaust ports. A fifth transfer port is not considered necessary due to the generous size of the existing four.
Light alloy pistons have one Dykestype piston ring and the usual needle bearings and roller bearings with aluminum cages are employed at the wrist pin and lower end of the rods. Four heavy-duty bearings, two at the sides and two in the middle, support the crankshaft.
A compression ratio of 12.5:1 (uncorrected) is used on each machine and the bore/stroke dimensions are 56x50mm for the 250 and 64x54mm for the 350. Mikuni carburetors with venturi diameters of 30 and 34mm are used.
A chrome-moly double cradle frame has proven adequate in spite of the fact that there are four rubber mountings for the engine. An eccentric ring at the swinging arm pivot is used for rear chain adjustment, and the wheelbase length may be altered by choosing one of two holes in the spindle ring.
Front suspension is by Ceriani and Girling shock absorbers control the rear wheel’s up and down movement. Dunlop tires (3.25-18 front and 3.50-18 rear) are fitted to Boranni rims and Ceriani double leading shoe brakes stop the light machines which weigh 238 lb. and 242 lb. respectively.
Concluding on a happy note for the Cafe Racer is the fact that both machines have been designed for roadster conversion: the engines already have provisions for fitting a kickstarter pedal and an AC generator! [oj
