Private company

Siddeley-Deasy

British automobile, aero engine & aircraft company

The Siddeley-Deasy Motor Car Company Limited was a British automobile, aero engine and aircraft company based in Coventry in the early 20th century. It was central to the formation, by merger and buy-out, of the later Armstrong Siddeley Motor and Armstrong Whitworth Aircraft companies.2 It ceased to exist in 1919.1 Wikipedia text CC BY-SA 4.0

Last updated Oct 20, 2026.

Overview

1906Founded1
  • Industryautomotive industry1
  • HeadquartersCoventry1
  • Legal formpublic limited company1

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Siddeley Deasy 1913 5134500942
Photo: Howard Burrows, CC BY-SA 2.0, via Wikimedia Commons4

The Siddeley-Deasy Motor Car Company Limited was a British automobile, aero engine and aircraft company based in Coventry in the early 20th century. It was central to the formation, by merger and buy-out, of the later Armstrong Siddeley Motor and Armstrong Whitworth Aircraft companies.2

The story

The Deasy Motor Car Manufacturing Company Limited was founded by Henry Hugh Peter Deasy in the factory that had previously been used to manufacture Iden cars. Deasy left in 1908 following disagreements with his Chief Engineer. In 1910 J D Siddeley took up the appointment of managing director having moved to Deasy in 1909 from managing Wolseley. The shareholders were so pleased with his success that on 7 November 1912 they unanimously agreed to change the company's name to The Siddeley-Deasy Motor Car Company Limited. Siddeley's name had been added to the product's radiator in 1912.

Siddeley-Deasy grew rapidly using Rover chassis and Daimler and Aster engines. They also established a separate brand, Stoneleigh, at first, in 1912, by mounting a different radiator and bonnet on a BSA 13.9 hp product but in the 1920s a quite separate car was produced and sold under the Stoneleigh name. Described as a nippy performer its quarter-elliptic springs gave it a curious bounding motion. The Wholesale Cooperative Society took them as vans.

From Wikipedia’s article on Siddeley-Deasy, shared under CC BY-SA 4.0.3

Products and brands

Cars

The J D Siddeley Type Deasy Car for 1911 Four models with seven standard styles of coachwork

"The highest-grade British car of moderate power"

12-16 hp 4-cyl poppet valve 75x110 = 1944 cc £385

14-20 hp 4-cyl poppet valve 80x130 = 2614 cc £375

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16-20 hp 6-cyl sleeve valve 90x130 = 4962 cc £445 (the Silent Knight engine was introduced in late 1911)

18-24 hp 6-cyl sleeve valve 90x130 = 4962 cc £685

1912 Stoneleigh 13.9 hp 4-cyl sleeve-valve 75x114 = 2015 cc

Aero-engines

Siddeley Puma

Siddeley Tiger

Aircraft

Siddeley-Deasy R.T.1

Armstrong Whitworth Siskin

Siddeley-Deasy Sinaia

From Wikipedia’s article on Siddeley-Deasy, shared under CC BY-SA 4.0.3

Acquisitions

  1. Succeeded by Armstrong Siddeley1

Timeline

  1. 1919Dissolved Dissolved1
  2. 1906Founded Founded1

Sources

The numbers on this page point here. Each one links to the record it came from.

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