1963 Matchless G12 CSR 650cc Motorcycle

Description

The Matchless G12 was one of the last motorcycles under the Matchless name and was also produced as the AJS Model 31 by the same company. Designer Phil Walker of AMC knew that it had to be a 650 cc (39.7 cu in) but wanted to use as many parts from the Model 11 as possible. The cylinders could not be bored out further so the stroke was lengthened from 72.8 to 79.3 mm (2.87 to 3.12 in), resulting in a capacity of 646 cc (39.4 cu in). This 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.The Matchless G12 CSR designation officially stood for "Competition/Sprung/Roadster," the same nomenclature used with the G80 and G50 models. It is mistakenly referenced as Competition/Sport/Road. It was dubbed the Coffee Shop Racer by its rivals With its distinctive two-into-one 'siamese' exhaust system and upgraded camshafts the CSR was a high performance motorcycle with what was, at the time, a high compression ratio of 8.5:1, distinguishing it and the G12 CS from the other G12 models, having a 7.5:1 compression ratio. The factory diverted Chief Engineer Jack Williams from AJS 7R development to address the problems with leaks and vibration and modify the bikes for racing - with the result that Ron Langston and Don Chapman won the prestigious Thruxton 500 long distance endurance race on the AJS Model 31 counterpart to the G12. Encouraged by this victory in 1963 the G12 CSR gained the name Monarch and twin carburetors (the AJS version was renamed the Hurricane).