|Engine||500cc Single –cylinder|
The A.J.Stevens Company was founded in 1909 by the four Stevens brothers, with motorcycle production commencing the following year. In the Twenties, AJS made a name for itself with a new OHV single-cylinder engine that helped the Wolverhampton outfit score wins in the 1921 and 1922 Junior TTs. Despite expanding into light cars and other products, AJS were struggling by the late Twenties and the manufacturing rights for their motorcycle production passed to Matchless in 1931 before becoming part of the Associated Motor Cycles group in 1938. In 1935. The AJS 500cc Model 18 and AJS Model 18S are 500 cc British motorcycles almost identical to the Matchless G80 and both were produced in the same Associated Motor Cycles (AMC) London factory from 1945 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 Model 18 was developed from a design from the 1930s, despite which it was still being manufactured 30 years later. The AJS was updated when a springer frame rear suspension was made available for 1949, to become the Model 18S. The suspension was a vast improvement on the rigid rear end - which had given a bouncy ride. Each of the two 'Candlestick' shocks held only 50 cc of SAE 20 weight oil and were replaced by the larger diameter 'Jampot' shocks on the 1951 version. The engines compression ratio of 5.9:1, necessary 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 18:s 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 competition models had also an alloy cylinder (with steel liner in it), where the barrel fins went all the way to the base. The AJS and Matchless singles ceased when AMC merged with Norton as all production was transferred to twins cylinder bikes.