No one at Longbridge in 1951 could have had any inkling that their new small engine would live until the 21st Century and become one of the most common and respected power units in the world.

The Austin A30 economy car, and its all-new engine, was one of the last products of the independent Austin company. Less than a year after it debuted Austin became part of the British Motor Corporation (BMC) and the small engine became the BMC A Series.

The A Series’ economy, durability and excellent performance for its size meant that it quickly spread throughout the BMC range, migrating into the Morris Minor almost the second BMC was formed. Soon numerous different capacities were being made consecutively to cope with the engine’s remarkable breadth of capabilities. In 848cc form the A Series was the only power unit compact and efficient enough for use in the remarkable Mini, while the best-selling ADO16 range used the 1098cc and 1275cc versions. The Austin-Healey Sprite and the Mini Cooper made the A Series a prodigious race and rally winner while countless racing specials used these cheap, strong and easily-available engines to good effect.

It’s hard to pin down exactly how many A Series engines were made, but the figures must stand at over ten million and it powered all of the top three best-selling British cars of all time.

The main issue with the A Series is that it simply wears out with age. Most get tired at between 100,000 and 150,000 miles and the bores are the first to go, so watch for plumes of blue smoke. The bottom end can be over-taxed too, shown by rumblings under load and low oil pressure. Clean coolant with a proper mix of anti-freeze is a must, and listen for rattles from the valve gear indicating a worn rocker shaft.

These engines have simple needs: Oil and filter changes every 4000 miles, with a good quality 20w50 grade oil, will ensure long life. Also, the tappet clearances should be checked and adjusted every 12,000 miles. The blocks are prone to silting up so flushing the cooling system annually is a good idea. The A Series does not take well to running on unleaded, making an additive or a cylinder head with hardened valve seats a must.

The most powerful A Series available from the showroom was the 93 horsepower 1275cc A-Plus in the MG Metro Turbo (1982-1990), which used a Garrett T2 turbo-charger to great effect. Tuners and racers have extracted a reliable (if rather peaky) 100-plus horsepower from a 1275cc (or special 1380cc) A Series and even more power has been gained through forced induction. The three-bearing crank is the ultimate limiting factor.

The A Series only ended production in October 2000 because the Mini did – the engine itself met all current and upcoming regulations. Rover and BMW had invested heavily in fitting electronic fuel injection to the A Series in the ‘Nineties and keeping it abreast of noise and emissions rules.

The key to the A Series’ success was its cylinder head, designed by Harry Weslake. With its heart-shaped combustion chambers and compounded ports this offered extremely efficient combustion that could return either superb fuel economy or class-leading power. The all-iron construction and generous bearing surfaces made the engine tough and capable of tolerating the odd bit of overheating or oil starvation too.

British Leyland realised that it would be more cost-effective to overhaul the A Series rather than design a whole new unit that would be only marginally better. The result was the A-Plus Series, launched in 1980 with revised manifolds, stronger internals, lighter pistons and other detail improvements that allowed the engine to live for another 20 years.

BMC developed a diesel version of the 948cc A Series to power the BMC Mini Tractor – it produced all of 15 horsepower. A petrol version of the same capacity was used in the Centurion tank as an auxiliary power unit.

Contrary to popular belief the Nissan A Series engine of 1967 is not a clone of the BMC design – it uses many of the same design features (including the Weslake combustion chambers) but has no interchangeable parts.