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AJS Model 18

AJS motorcycles

The AJS Model 18 is a British motorcycle almost identical to the Matchless G80 and both were produced in the same Associated Motor Cycles (AMC) London factory from 1949 to 1966. These bikes represent the end of the era of big British singles, as when AMC merged with Norton production concentrated on twins.


The AJS 18 was developed from a design from the 1930s despite being manufactured 30 years later. The AJS was updated when a spring frame (sprung-heel) rear suspension was made available. The suspension was a vast improvement on the rigid rear end which gave a bouncy ride but the wheel travel was still limited.

AMC then copied Velocette's twin-shock swingarm rear suspension design for its top AJS and Matchless singles and twins. Shorter vertical shock absorber units were introduced in 1949. One of these 'Candlestick' shocks held only 50cc of SAE 20 weight oil. They were prone to leaks and were replaced by the 'Jampot' shocks on the 1951 version.


The engine was released with a compression ratio of 5.9:1, necessary because of because of the low quality fuel available in Britain immediately after the War. British singles were designed to make the best of the fuel available. Post war petrol rationing continued until 1950 and it was several years before performance fuels were generally available in the UK. The Model 18's low compression did mean it was easy to start and The model 18S had better performance and fuel economy than the fast cars of the time.

By 1951 the model 18 had an alloy cylinder head and the barrel fins went all the way to the base. The magneto was moved in front of the cylinder on the AJS 18 for 1952. The earlier model did not have a magdyno - the magneto was directly above the dynamo behind the cylinder. The leaky pressed-steel primary chain-case first appeared in 1953, the compression ratio was increased to 7.3:1 in 1956 and in 1958 an alloy cover primary chain-case became available.

The unreliable 'jampot' shock absorbers were replaced with Girling shocks in 1956 and in 1957 AMC switched from Burman gearboxes to their own make. In 1960 the model 18 gained a duplex frame


The 500cc "long stroke" AJS 18CS was produced from 1951 through 1955. In 1956 it was replaced by the shorter stroke, larger bore models that used the same numeric codes. The bore of these early models was 82.5mm (3.25"), while the 1956 through 1966 "short stroke" (final version) models had a bore of 86mm. The C was for Competition (Scrambles) and the S for Suspension (not a rigid rear frame).

Engine numbers usually start with the year of production, followed by model designation, and completed with the production number of the motorcycle.

The AJS and Matchless singles were doomed when AMC merged with Norton as all production was transferred to twins cylinder bikes - so the short-stroke Model 18 is the end of an era.