| MODEL | MATCHLESS G-12 1958 |
| Year | 1958 |
| Engine Type | Twin, four-stroke |
| Displacement / Capacity | 646 cc / 39.42 cu in |
| Bore X Stroke | — |
| Compression Ratio | — |
| Cooling System | Air |
| Max Power | 25.5 kW / 35 hp @ 6500 rpm |
| Max Torque | — |
| Ignition | — |
| Induction | Amal carburettor |
| Start System | — |
| TRANSMISSION SPECS | |
| Lubrication | — |
| Gear Box | — |
| Clutch | — |
| Final Drive | Chain |
| CHASSIS SPECS | |
| Frame | — |
| Front Suspension | Telescopic |
| Rear Suspension | Swingarm with coil-over shocks |
| Front Brakes | Drum (expanding) |
| Rear Brakes | Drum (expanding) |
| DIMENSION SPECS | |
| Overall Length | — |
| Overall Width | — |
| Overall Height | — |
| Seat Height | — |
| Wheelbase | — |
| Ground Clearance | — |
| Dry Weight | 180 kg / 396.8 lb |
| Wet Weight | — |
| WHEEL SPECS | |
| Front Tyre | — |
| Rear Tyre | — |
| FUEL SYSTEM | |
| Fuel Capacity | — |
| Fuel Consumption | — |
| Top Speed | — |
The G-12 was the large parallel-twin road model in the Matchless range from the late 1950s into the mid 1960s, a sporty 650 with an Amal carburettor, telescopic forks and a swingarm frame with twin shocks. The database credits Phil Walker as designer. The page adds that the model was also called the Monarch and was sold as the AJS 31 by the same company.
