The AJS Model 31 was a British motorcycle made by Associated Motor Cycles at the former Matchless works in Plumstead, London.

[2] Designed by Phil Walker, AMC knew that it had to be a 650cc but wanted to use as many cycle parts as possible from the preceding 600cc Matchless G11, which had been badge-engineered as the AJS Model 30.

[1] In 1959 the Model 31 was redesigned and modernised as the De Luxe, with a new full cradle tubular duplex frame and a new cylinder head.

The AJS Model 31 CSR designation officially stood for Competition / Springer / Road but it was dubbed the Coffee Shop Racer by its rivals.

[3] With its distinctive two into one "siamese" exhaust system and upgraded camshafts, the CSR was a high-performance motorcycle but still prone to leaks and vibration.