The A40 engine was originally drawn up to be produced in two capacities of 990cc and 1200cc to fall into the Eight and Ten tax horsepower classes.

Therefore, only the 1200cc version was built but could not be increased in capacity any further since it was already nearly at the maximum bore and stroke dimensions permitted by the design of the block casting.

The long stroke, narrow bore characteristics of engines designed for the British taxation system remained.

This was to allow room for enlargement of the cylinder bore to provide the larger capacities foreseen by BMC.

Originally of approximately 1.2 Litre capacity, later displacements ranged widely from 1.2 L to 2.4 L, the latter being a six-cylinder variant which was only produced in Australia.

This included a version of the engine built under license in India by Hindustan Motors for its Ambassador series of cars.

The engine was of conventional construction with a one-piece crankcase and cylinder block in cast iron with the crankcase extending down to the lowest level of the main bearing caps; with a cylinder head, also usually in cast iron, and a sump made from pressed steel for rear-wheel drive vehicles.

The 1.5–litre diesel engines were made in India by Hindustan Motors for many decades until the production of the Ambassador was phased out in 2013.

The 1.5 B-Series would also form the basis of the 1958 Perkins 4.99 / 1.6 L (1,622 cc) diesel, which was fitted in cars, boats and commercial vehicles including London Taxis like the Beardmore Mk7 Paramount taxi, Ford Thames 400E, early Ford Transits, Bedford CA, Vauxhall Victor,[9] Hillman Minx,[10] Standard Ensign and Moskvitch 407.

In the mid-1960s the 4.99 was upgraded to become the 4.107 / 1.8 L (1,760 cc) and later the classic 4.108 / 1.8 L (1,760 cc), powering the Bedford CF, Commer FC, Alfa Romeo F12/A12, Alfa Romeo Giulia, SEAT 131 and Moskvitch 408 as well as being used extensively in marine applications, farm equipment and Mustang/OMC skid-steer loaders.

The engine block was cast iron, but the crossflow eight-port cylinder head was of aluminium alloy.

This engine gained a reputation for being unreliable in service, especially in the high-compression version which needed high-octane fuel, but this has now been largely overcome.

The "Blue Streak" was an inline-6 development of the B series, adding two extra cylinders to create a 2.4 L (2,433 cc) engine.

There was also a diesel version of this capacity, used in the Leyland Sherpa van with a power output of 56 bhp (42 kW) at 4,250 rpm, and built under license in Turkey for many years.

[13] There was a 2.4 L (2,433 cc) six-cylinder unit, named "Blue Streak", which was used in the Australian Austin Freeway and Wolseley 24/80.

[14] There was sometimes a country indicator after the first part of the code, e.g. "18V-Z" was used for some United States (except California) MG MGB engines.