UNDER THE SPIKES OF THE PORCUPINE
EIGHT AJS 500cc parallel-twin GP racers were built from 1947 to 1954, four each of two different types: the E90S with its cylinders raised 10º from horizontal, and the E95 introduced in 1952 with cylinders inclined at 45°.
A total of 12 engines were manufactured, four each in 1947 (the original spiky-finned design), 1951, and 1953. While externally these differed architecturally, inside they all shared a common format, as well as major components. The 498cc motor’s 68mm x 68.25mm ‘square’ engine dimensions were unusual and far-sighted in an era of longstroke engines, as was its unit-construction design with gear primary drive. Other British race bikes still employed separate gearboxes and open primary chains.
From ’51 on, the AJS was shorn of spikes, with more conventional fins (though the ‘Porcupine’ nickname remained). Heavily finned and deeply-spigoted separate aluminium cylinders are surmounted by a one-piece alloy dohc cylinder head, whose hollow twin forged roller-bearing camshafts are driven by a train of eight gears running on roller bearings up the right side of the engine in a Y-shaped case, with an outrigger plate supporting the ends of the spindles.
A ninth, half-time, spur gear drives the high-pressure oil pump, with main feeds to the centre main bearing and cam boxes, to also lubricate the big-ends, and each cam face. The same pinion also drives the Lucas magneto mounted on the engine cases, via a short chain.
The pressed-up three-bearing crankshaft with 360º throws (both pistons rise and fall together) revolves backwards, like most modern Motogp engines, and is mounted on a double-flanged plain centre main bearing, with caged-roller outer bearings. Originally, the crankshaft was a forged one-piece unit, with effectively no flywheels, a spin-off from the original blown motor, where compressor loading would have replaced their inertia effect. The resultant abrupt extremes of engine acceleration and deceleration produced massive loadings in both directions on the Lucas magneto’s two-piece drive shaft, with consequent fractures which plagued the AJS through its early career, until the shaft was replaced by a chain which absorbed shock better.
Hepolite full-skirt forged aluminium pistons are fitted, mounted on forged, bolted-up RR56 aluminium conrods, whose split big-ends ran on white-metal plain Vandervell bearings. The two-valves-per-cylinder were 7/8in (22mm) nimonic inlets and 5/8 in (16mm) sodium-cooled exhausts, all with quite thick stems and set at an 87º included angle, controlled by fully enclosed overlapping hairpin springs.
Twin straight-cut spur gears comprise the primary drive, with final drive from the four-speed, all roller-bearing Burman gearbox exiting on the opposite side, on the right of the bike. The large-diameter dry clutch runs at a fast 0.7 times engine speed, a hangover from the substantial torque expected from a supercharged motor. The twin 11/8in (32.5mm) Amal GP carbs are rubber-mounted to the inlet stubs, which deliver an extra four degrees of downdraught. Fuel flows from the massive 6½-gallon (29.4 litre) fuel tank to a remote float positioned beneath and behind the gearbox.
The E90 tubular-steel frame sees the engine carried quite low down, in a double-cradle design welded up from both round and oval-section tubing. The AMC trademark fork and shocks offer a mere three inches (75mm) of movement front and rear. The front 19in wheel carries the same 8.25in (210mm) twin-leading-shoe drum brake as the lighter, slower 350cc 7R single, with cast magnesium hubs.
The wheelbase is shorter than it looks at 56½in (1435mm), while the dry weight of 335lb (152kg) was no lighter than the half-faired version of the Gilera 500/4, with its bigger brakes and twice as many cylinders.