AJS 31

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The AJS 31 is a motorcycle from the British manufacturer Associated Motor Cycles (AMC).


Model 31

The AJS 31 is the further development of a four-stroke parallel twin with 500 cm³ that has been built since 1949. To secure sales in the United States, a version drilled to 600 cc appeared in 1956. Because a 650 cc machine was desired, AMC decided to design an engine with a new crankshaft. The motorcycle appeared in model year 1959 under the name AJS 31. Matchless , which is also part of the group , released the model as the G 12.

Model 31 had a compression ratio of 7.5: 1 and an AC system including an ignition distributor.

Model 31 De Luxe

The motorcycle had magneto ignition and manual ignition adjustment as well as a quick-release axle in the rear wheel and was characterized by more ornaments.

Model 31 CS

The machine was intended for off-road sports and was delivered with studded tires. It also had a higher compression and was lighter.

CSR model

The model corresponded to the 31 CS, but was designed for the road, with correspondingly different tires. The motorcycle reached a top speed of 177 km / h. Strong vibrations, which extended over the entire speed range, were disadvantageous.

Due to the large stroke and heavy pistons, all models tended to break the crankshaft.

Technical specifications

Parameter Data of the AJS 31 De Luxe
Displacement 646 cc
Bore × stroke 72 x 79.3 mm
power 40 hp at 6000 rpm
compression 7.5: 1 (optionally 8.5: 1)
Carburetor an Amal type 389, opening 28.5 mm
ignition Magneto ignition
lubrication Dry sump lubrication
landing gear lugged single-tube frame with double girders, telescopic fork at the front, strut swing arm at the rear
brake Drum brake front and rear
tires from 3.25 S 19 TT, rear 3.50 S 19 TT
Empty weight 195 kg
consumption 5 l per 100 km
Top speed 155 km / h


  • Oldtimer Markt December 2010 issue, pp. 190–195