The AJS Model 31 was also produced as the Matchless G12 by the same company.[2] Designer by Phil Walker, AMC knew that it had to be a 650cc but wanted to use as many cycle parts from the Model 11 as possible. The cylinders could not be bored out further so the stroke was lengthened from 72.8mm to 79.3mm, resulting in a capacity of 646cc. The meant developing a new crankshaft and primary chain case, which also provided the opportunity to add a Lucas alternator. Volume production began in September 1958.

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