The Aermacchi
Anatomy Of A 250cc Racer: THE AERMACCHI
CARLO PERELLI
DOES ANYONE remember the all-enclosed “Chimera” 175cc launched by Aermacchi at the 1956 Milan Show? It was an interesting design from a factory which had entered our field soon after the second World War, producing “big wheel” scooters and three-wheeled trucks (Aermacchi was one of the most famous fighter plane producers of Italy but, of course, after the war they had to look for another activity).
Well, to tell the truth, the Chimera wasn’t very successful either in Italy or abroad so Aermacchi tried a more orthodox way, producing the “naked” version in touring and sport trim, which soon became popular. Then a 250cc version was “overbored” from the 175cc, increasing the bore size from 61mm to 72mm (Ed. Note: Oddly enough, these dimensions are not those of the standard H-D Sprint), but leaving the stroke unaltered at 61mm.
Both these machines, competing in national “production racer” categories, were extremely successful, and plans were made to have a try in international Grand Prix type racing. The debut came at the 1960 German G.P. and was quite promising, so Aermacchi decided to build small batches of 250cc racers to be sold to private riders, following the same policy of the English factories, which has since proved quite sound.
At the moment Aermacchi is the only Italian factory building production racers and orders for these machines come from all over the world to the modern plants on the shores of the Varese lake in Northern Italy. The 250cc Aermacchi racers are particularly favored because their performances are excellent in spite of their simplicity; they cost only 500,000 Liras (in Italy) and also feature ease of maintenance and a sturdy structure. Spare parts are always available at moderate prices and tuning is not particularly complicated.
The Aermacchi features a horizontal cylinder and a single beam frame; this makes for a lower center of gravity and also reduces frontal area. The power unit, with pushrod-operated ohv, transmits 29 BHP at the rear wheel @ 9800 rpm and has excellent torque from 6700 to the maximum rev range. Moreover, a good pulling power is on tap all the way from 4000 rpm, one of the machine’s best features.
Feeding is through a 30mm Dellorto carburetor with separate and adjustable float chamber. Lubrication is wet sump type, with gear pump (the sump has a capacity of 4 pints and the use of pure castor oil is recommended). Ignition is by the usual coil and battery system, with manually operated advance, from 50 to 55 degrees before TDC; a Lodge RL plug takes care of the sparks.
If you are interested in more detailed data, here is the valve timing; inlet opens 79 degrees before TDC and closes 94 degrees after BDC; exhaust opens 98 degrees before BDC and closes 66 degrees after TDC; valve clearance is .007-in. for the inlet and .009 in. for the exhaust. And here are the various ratios: primary drive 2,091 (33/69 teeth); gearbox, first 2,263; second 1,757; third 1,378; fourth 1,126; 1 top. With the old four-speed gearbox, used until last year, the various ratios were of course farther apart and low gear was somewhat higher. The introduction of the five-speed gearbox has brought better performance to the machines, especially when competing on windy courses. The gearbox sprocket has 16 teeth for the “naked” machines and 18 teeth for faired models; the rear wheel sprocket has 48 teeth. With simple calculation you can work out the final drive ratios.
The complete power unit weighs 92 pounds; the machine, 207 pounds in naked form and 216 pounds with a fairing. Fuel tank capacity, 3.4 gallons; 2.75-18 (front) and 3.00-18 (rear) tires; 8.6-in. diameter brakes, the front one with two leading shoes. Recently Aermacchi has introduced the 350cc version, with 74x80mm bore and stroke dimensions.
