The 18HP was produced between 1921 and 1927, with a total production of 2,500 vehicles (which includes the 18 Mk2). The engines fitted were a six-cylinder 2318cc unit that were mated to a manual gearbox. The 18HP model was released at the Olympia Show in 1921. It was a contemporary of the 30HP, only smaller and cheaper.  It was in fact a 30HP, reduced in size but not in quality. 

The 18HP had a wheelbase of 10’ and a track of 4’ 3”. The  6 cylinder motor, with an RAC rating of 17.9 hp, had two 3-cylinder castings with aluminium water jacket coverings, overhead valves and magneto ignition.  The chassis was equipped with stepboards, wings, engine starter and electric lighting complete with 5 lamps!  The price of the chassis was £575 which included speedometer, electric klaxon horn and spare tyre.

A number of body styles were available, including open touring car, saloon to seat 4 and a  laundaulette to seat 4 or 6.  In 1922 the Duke of York (later King George VI) ordered two cars based on the 18HP chassis with all timber coachwork and wood paneling. One was a rather low, pale coloured sports model with a hide leather top, whilst the other was a formal State Car.

Armstrong Siddeley 18HP Mk2

The 18 Mk2 was produced between 1925 and 1926, with a total production of 2,500 vehicles (which includes the 18HP). The engines fitted were a six-cylinder 2872cc unit that were mated to a manual gearbox. The 18 Mk2 was introduced in 1925 with a new 2,872cc engine having a bore/stroke of 73 x 114.29mm and rated at 20HP. The 18HP still remained in parallel production until 1927.

In an Autocar road test of December 18, 1925, the author wrote "The engine pulled smoothly and sweetly at all normal touring speeds the central gearchange works lightly, but as is a habit of this make, we have noticed, to need a particularly long wait when changing with a double-clutch movement; single clutching is apt to produce a slight crunch as the gear goes in ..... altogether , a thoroughly roadworthy car offered at a very attractive price."

A contented owner wrote to the factory in July 1927 "It may interest you to know that last weekend I climbed Honister Pass from the Buttermere side in the Short 18 Saloon with five up. The pass is reputed to be 1 in 3 to 1 in 4 with about 12" of loose metal. It is marked at the bottom by the A.A. as being impractible for motorists."

In 1927 the 18 Mk2 was renamed the Short 20 and the Long 20 according to the wheelbase length.

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Armstrong Siddeley vehicles