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Matchless G12 (all models)

Make Model	Matchless G12 (all models)
Year	1958 - 66
Engine	Twin cylinder
Capacity	646 cc / 39.4 cub in.
Bore x Stroke	72 x 79.3 mm
Cooling System	Air cooled
Compression Ratio	G12/de Luxe: 7.5:1 CS/CSR: 8.5:1
Lubrication	Dry sump
Engine Oil	Hot: SAE 50 Cold: SAE 30 Extremely cold: SAE 20
Oil Capacity	2.5 L / 5.3 US pints
Exhaust	Twin, chrome
Fuel System	Amal 389, 1-1/8
Ignition	Lucas K2F
Starting	Kick
Battery	Lucas 6V, 12Ah
Spark Plug	G12/de Luxe/CS: KLG FE80, 14 mm thread, ¾ in. reach CSR: KLG FE220, 14 mm thread, ¾ in. reach
Transmission	4-Speed
Final Drive	Chain, 97 links
Internal Gear Ratios	1st 2.56 / 2nd 1.70 / 3rd 1.22 / 4th 1.00:1
Maximum Power	26 kW / 35 hp @ 6500 rpm
Frame	Double cradle
Front Suspension	AMC teledraulic fork
Rear Suspension	Swingarm with twin shocks, Girling
Front Brakes	Drum, 7in. sls
Rear Brakes	Drum, 7in. sls
Wheels	Steel, wire spokes
Front Tyre	3.25 x 19 in.
Rear Tyre	3.50 x 19 in.
Dry Weight	180 kg / 396 lbs
Fuel Capacity	17 L / 4.5 US gal
Average Consumption	4.3 L/100 km / 23.4 km/l / 55 US mpg
Top Speed	153 km/h / 95 mph

Developed in 1958 specifically to capture the potentially lucrative US market, the last G12 was produced in 1966.

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 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.

Matchless G12 De Luxe
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. Capable of 100 mph (160 km/h) performance, the G12 became popular with the American market.

Matchless G12 CS
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
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).