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 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.