The design of this unit were derived from the much larger OHV engine developed for the wartime Austin K5 military truck, which used a 3995cc straight-six engine (minus two cylinders this engine became the 2660cc four-cylinder engine used in the Austin A90 Atlantic and the Austin-Healey 100). The K5 engine was broadly an Austin version of the 3519cc engine built by Bedford for its military trucks, and in turn the Bedford engine was similar in design to the Chevrolet 235 straight-six engine - Bedford and Chevrolet both being part of General Motors. The common features found on all these engines, from the Chevrolet 235 to the Austin 1200cc, include the design of the valve gear and the cylinder head, especially the siamesed cylinder head ports.