The A series of internal combustion engines started in the late 1940s, without any particular fanfare. Little did Morris know what they'd started.
The A series engines all share certain characteristics:
They are all four cylinder overhead valve four-stroke petrol engines with capacities between 803cc and 1275cc. They were designed by W. V. Appleby.
The A engine was originally used in the Morris Minor, in 803cc form, and later 948 and 1098cc versions. It replaced an earlier sidevalve unit, and the 1098cc version produces roughly 60 brake horsepower in untuned form.
The successor to the Minor was, of course, the famous Mini, which was the first transverse engined car; mounting the engine sideways, with the cylinders across the engine compartment, allowed a much shorter hood. The Mini also had revolutionary self-levelling Hydrolastic suspension, though it was eventually removed because passengers complained that it made them feel sick. Even more revolutionary was that the Mini had driven front wheels.
The Mini had an 850cc A series engine turned sideways, with the gearbox mounted in the sump to save space. This is why all Minis leak oil. Later a Mini 1000 and even a Mini 1100 were produced, the latter with an optional 5-speed gearbox. Post-'Mini Mayfair' Minis had an A+ series engine.
Of course, the A series engines were extremely tweakable, being such simple designs. The Cooper company took Mini engines, bored them out to 1275cc, uprated many components, ground out the valves, and generally did unto them to turn the Mini into a rally machine. (Mini Coopers are most famous for their role in The Italian Job, which rocks out.)
Austin bought Morris, looked unto Sir Alec Issigonis, designer of both the Minor and the Mini and saw that he was right all the time; when he had said 'the future is overhead cams', he was right. And so, they designed the K series engine; based on the same principles and precepts and even some of the same tooling as the A series; a four-cylinder compact small-displacement engine.
Meanwhile, the A series was not quite dead. The Mini design was looking a bit 1960s; while the mechanics were all standing up well, and the reliability and performance were fine, the body and chassis were looking distinctly dated. The Mini Metro was the result; a modern-looking car with essentially identical internals to the Mini. Right down to the A series engine. Hurrah for the A series!
Later models of Metro were produced, under the ownership of Rover, which had K series engines; they are the ultimate sleeper car- looking like a tiny shopping car, but managing in excess of 160 mph in tuned form. Mental.