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. Before the A40 was launched, the British government removed the tax horsepower system in favour of a flat tax rate making the smaller capacity redundant. 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.