In April 1919 Siddeley-Deasy was bought out by Armstrong Whitworth Development Company of Newcastle upon Tyne and in May 1919 became Armstrong Siddeley Motors Ltd a subsidiary with J. D. Siddeley as Managing Director. In 1927, Armstrong Whitworth merged its heavy engineering interests with Vickers to form Vickers-Armstrongs. At this point, J. D. Siddeley bought Armstrong Siddeley and Armstrong Whitworth Aircraft into his control. In 1928, Armstrong Siddeley Holdings bought Avro from Crossley Motors.
Armstrong Siddeley manufactured luxury cars, aircraft engines, and later, aircraft. In 1935, J. D. Siddeley's interests were purchased for £2 million by Tommy Sopwith owner of Hawker Aircraft to form Hawker Siddeley, a famous name in British aircraft production. Armstrong Whitworth Aircraft became a subsidiary of Hawker. The aviation pioneer Thomas Octave Murdoch Sopwith - Tommy, later Sir Thomas, Sopwith - became chairman of Armstrong Siddeley Motors, a Hawker Siddeley subsidiary.
Armstrong Siddeley was merged with the aircraft engine business of Bristol Aero Engines that had developed the Olympus engines for the TSR 2 (that engine was subsequently developed for used in Concorde) to form Bristol Siddeley as part of an ongoing rationalisation of the British aerospace sector. Armstrong Siddeley produced their last cars in 1960. Bristol Siddeley and Rolls-Royce merged in 1966, the latter name subsuming the former.
In June 1972, Rolls-Royce (1972) Ltd. sold all the stock of spares plus all patents, specifications, drawings, catalogues and the name of Armstrong Siddeley Motors Ltd to the Armstrong Siddeley Owners Club Ltd.
However, the name survived with Hawker Siddeley Aviation and Hawker Siddeley Dynamics joining with others to become BAe - British Aerospace, and with further mergers has now become BAE Systems the premier defence contractor, which among other things builds Aircraft Engines, Aircraft, and Aircraft carriers.
Throughout the 1920s and 1930s, Armstrong Siddeley produced a range of low- and mid-power aircraft radial engines, all named after big cats. They also produced a tiny 2-cylinder engine called the Ounce, another name for the snow leopard, for ultralight aircraft.
The company started work on their first gas turbine engine in 1939, following the design pioneered at the Royal Aircraft Establishment by Alan Arnold Griffith. Known as the "ASX" for "Armstrong Siddeley eXperimental", the original pure-turbojet design was later adapted to drive a propeller, resulting in the "ASP". From then on, AS turbine engines were named after snakes. The Mamba and Double Mamba were turboprop engines, the latter being a complex piece of engineering with two side-by-side Mambas driving through a common gearbox, and could be found on the Fairey Gannet. The Python turboprop powered the Westland Wyvern strike aircraft. Further development of the Mamba removed the reduction gearbox to give the Adder turbojet.
Another pioneer in the production of the RAE engine design was Metrovick, who started with a design known as the Metrovick F.2. This engine never entered production, and Metrovick turned to a larger design, the Beryl, and then to an even larger design, the Sapphire. Armstrong Siddeley later took over the Sapphire design, and it went on to be one of the most successful 2nd generation jet engines, competing with the better-known Rolls-Royce Avon.
The company went on to develop an engine - originally for unmanned Jindivik target drones - called the Viper. This product was further developed by Bristol Siddeley and, later, Rolls-Royce and was sold in great numbers over many years. A range of rocket motors were also produced, including the Snarler and Stentor. The rocket development complemented that of Bristol, and Bristol Siddeley would become the leading British manufacturer of rocket engines for missiles.