Matchless G12

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Matchless G12
Matchless G12 1959.JPG
Manufacturer Associated Motorcycles Plumstead, London
Also called 'Monarch'
Production 1958 - 1966
Predecessor Matchless G11
Engine 646 cc (39 cu in) air cooled twin
Power 35 bhp (26 kW) @ 6,500rpm
Transmission Four speed gearbox to chain final drive
Brakes drum brakes
Weight 396 pounds (180 kg) (dry)

The Matchless G12 is a British motorcycle made by Associated Motorcycles at the former Matchless works in Plumstead, London. Developed in 1958 specifically to capture the potentially lucrative US market, the last G12 was produced in 1966.[1]


Matchless G12[edit]

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.[2] Designer by Phil Walker, 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 chaincase, which also provided the opportunity to add a Lucas alternator. Volume production began in September 1958.[1]

1960 Matchless G12 in use

Matchless G12 De Luxe[edit]

In 1959 the G12 was redesigned and modernised as the De Luxe with a new full cradle tubular duplex frame and a new cylinder head. Vibration had always been a problem, so the crankshaft was upgraded to nodular iron.[1] Capable of 100 mph (160 km/h) performance, the G12 became popular with the American market.

Matchless G12 CS[edit]

The Matchless G12 CS (sometimes referred to as the CS X) was an off-road version of the Matchless G12 with improved ground clearance and a slightly upswept exhaust and an 8.5:1 compression ratio. It was not a trials competition machine but instead aimed at the 'desert racer' market in the US. While never as popular as rivals Triumph and BSA 650's in Open Class desert competition, the Matchless was powered by a torquey and reliable motor and was suspended with the superior Teledraulic front fork.

Matchless G12 CSR[edit]

The Matchless G12 CSR designation officially stood for "Competition/Sprung/Roadster," the same nomenclature used with the G80 and G50 models.[3] It is mistakenly referenced as Competition/Sport/Road. It was dubbed the Coffee Shop Racer by its rivals.[4] 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 carburettors (the AJS version was renamed the Hurricane).[5]

Matchless G15/45[edit]

In 1964 the CSR gained Norton brakes and forks, and the following year a Norton Atlas 750 cc (45.8 cu in) engine replaced the trouble prone AMC unit. In 1962 and 1963, AMC had produced a limited run of 212 touring motorcycles of 738 cc (45.0 cu in) displacement using an enlarged version of the G12 AMC-designed engine, but these bikes—which were given the model designation "G15/45"—proved even less reliable than the 650 cc (39.7 cu in) version when ridden hard. So AMC's original plans to develop a 750 cc desert racer version were abandoned. If successful, this could have saved the company as there was a huge un-met demand in the US for powerful desert racers to compete in endurance events.[6] By this time the company were in financial trouble and production ended in 1966.[1]

See also[edit]


  1. ^ a b c d Currie, Bob (1988). Classic British Motorcycles of over 500cc. Patrick Stephens Ltd. ISBN 1-85260-083-7. 
  2. ^ "Matchless G12". Retrieved 2008-12-14. 
  3. ^ Bacon, Roy. AJS & Matchless Postwar Twins 1948-1969. Niton Publishing. ISBN 978-0-9514204-7-8. 
  4. ^ Westworth, Frank. The British Classic Bike Guide. Haynes Publishing. ISBN 1-85960-426-9. 
  5. ^ Falloon, Ian. "Meet the twins" (PDF). Retrieved 2008-12-14. 
  6. ^ "The Atlas Hybrids". Retrieved 2008-12-14. 

External links[edit]