Matchless G12
Matchless G12 Background & History
The Matchless G12 was AMC’s (Associated Motor Cycles) lasst foray into the hotly-contested world of vertical twins in the late 1950s. Life as a motorcycle manufacturer in the 1930s was fairly predictable, everyone had a big OHV single-cylinder bike, or several that were at the heart of their marketing strategy. As World War 2 ended and the world began to return to normal, that was all about to change.
During the 1920s and 30s, motorcycles developed and improved dramatically. One basic engine layout ruled the era: single-cylinder machines. They started out as side-valve (flathead) designs, but soon graduated on to overhead valves (OHV) as performance steadily improved. By the mid-1930s, displacement and power output had nearly reached their practical limits at around 500cc and 25 horsepower. Adding more displacement, or extracting more power by revving the engines faster only led to horrible engine vibrations that were hard on engines and riders alike. Many solutions were attempted but nothing seemed to work.
This 1963 Matchless G12CS-R was the high-performance road version of the G12.
Vibration is the Enemy
Shortly after Jack Sangster bought Ariel Motorcycles he brought over a promising new designer named Edward Turner, who promptly designed the first Ariel Square four. When Ariel bought Triumph, Turner was put onto the task of solving the vibration problems the company, and the entire industry, was experiencing. After all Turner had taken a bold approach with the Square Four and it was one of the smoothest bikes in existence.
Like his Square Four design, Turner’s solution took a whole new direction. He split that 500cc of displacement into two parallel cylinders rising and falling together on a 360-degree crankshaft, but firing alternately. The new ‘vertical twin’ as it was dubbed was lighter, made more power, loved to rev, and didn’t have nearly the vibration penalty of the big singles. In 1937 Triumph launched the new engine in a new bike, the 500cc 1938 Triumph 5T Speed Twin. It was an instant hit and changed the entire motorcycle landscape. Suddenly, all the big singles had been outclassed. Most of the British motorcycle industry rushed to bring their own vertical twins to market.
Check out the 2-into-1 exhaust system of this 1963 Matchless G12 CS-R.
Vertical Twin Race is On
During this era, and beginning with the Speed Twin, Triumph ruled the roost, always setting the pace of innovation, always leading the way with newer, bigger, faster bikes. As quickly as the rest would catch up, Triumph would launch another bike that left them all in the dust. The Speed Twin did it first in 1938. Then, higher compression, hotter cams and a bigger carburetor were added, boosting horsepower, creating the ‘hot rod-version’ of the new twin, with the 1939 Triumph T110 Tiger. World War 2 broke out in 1939 and all civilian production ended until 1946, but when it did, Triumph was back with a vengeance.
In 1946 Triumph was back with their 500cc Speed Twin and the T100 Tiger. BSA was the first of Triumph’s rivals to hit the market with their own vertical twin, the 1946 BSA A7 Golden Flash. By 1948 Ariel had completed its entry into the fray, the 1948 Ariel KH. Then in 1949 Norton, Royal Enfield and Matchless/AJS all came out with their 500cc vertical twins, and the race was on, to build the best, the fastest, the most powerful, and of course the best-selling vertical twin on the market.
This is a 650cc 1963 Matchless G12 De Luxe.
Matchless Offers a 500cc Vertical Twin
Matchless was owned by AMC who also owned AJS and the parent company always treated them as ‘sisters’. When one got a new model the other one did also, virtually identical mechanically, but styled differently and given a different name, or rather a different alphanumeric designation. The new Matchless 500cc vertical twin was dubbed the G9 while the nearly identical AJS bike was called the Model 20. Both were displayed for the first time at the 1948 Earls Court Motorcycle Show (the first after the war), but it took until late summer of 1949 for the bikes to actually show up at dealerships. Both bikes were handsomely styled with lots of chrome and paint. Most of the cycle parts were from the Matchless/AJS parts bin.
1963 Matchless G12 650cc engine, timing side.
Matchless G12 Challenges the Triumph 650
But just as the rest of the market caught up with Triumph’s 500 twin, Triumph upped the ante again by boring and stroking its 500 out to 650cc with the 1950 Triumph 6T Thunderbird, and once again the industry scrambled to keep up. BSA brought out their 650cc A10, and all the rest did the same. Royal Enfield even went as far as offering a larger displacement than Triumph’s 650cc with the 700cc Royal Enfield Crusader twin. Matchless and AJS took until 1958 to come up with their own 650s, the Matchless G12 and the AJS Model 31.
1966 Matchless G12 engine, primary side.
Matchless G12 Engine Design
Designed by Phil Walker, the 650cc Matchless G12 (and AJS Model 31) was enlarged from the 500cc G9 by stroking it from 72.8mm to 79.3mm. The bore were already a big as they could go safely so it all had to be in the stroke. The net capacity was 646cc. This required a new crankshaft, primary chain case, and this was taken as an opportunity to add a new Lucas alternator, replacing the old dynamo. As 650s went back then, it wasn’t a blistering performer at 35 hp at 6500 rpm. Triumph’s 650s made more like 40.
This 1966 Matchless G12 represents the final year of production. Note the twin leading-shoe (TLS) front brake.
Matchless G12 Variants
In 1959 Matchless upgraded and redesigned the G12 into the G12 De Luxe with a new full cradle duplex frame and a new cylinder head. The crankshaft was upgraded to nodular iron in an effort to fight vibration. The G12 De Luxe was good for a 100 mph top speed. The Matchless G12CS (Competition Sport) came out in 1963 and was the off-load version with added ground clearance, a skid plate, 2-gallon tank, alloy fenders, a cool 2-int-01 Siamezed exhaust and a 8.5:1 compression ratio. Aimed at the lucrative US market, it wasn’t intended as a trials bike but rather a ‘desert sled’. The Matchless G12 CSR (Competition Sport, Road) was meant to be a road rocket. It too came with alloy fenders and a 2-into-1 exhaust system. A hot rod version of the Matchless G12 CSR was available with 10.5:1 compression, a hotter camshaft and twin carbs. These enhancement further stressed the already stressed bottom end, making an otherwise robust twin somewhat fragile. The last Matchless G12 was built in 1966. By this time, Matchless, AJS, AMC and most of the British motorcycle industry was on life support. Only Triumph, BSA and Norton would emerge from the ashes, and even they would only last a few more short years. BSA closed in 1972, Norton in 1975 and Triumph in 1983.
This 650cc 1958 AJS Model 31 is a nearly identical twin to the Matchless G12.
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