the even rarer version, below, it became that rarest of things: an oversquare A-series engine. Applications: 1963–1964 Austin/Morris Mini Cooper S, at 6000 rpm and at 4500 rpm 970 The Mini Cooper S next moved on to a version. It had the same bore as the 1071 cc Cooper S but used a shorter stroke. It was produced from 1964 to 1965. Applications: 1964–1967 Austin/Morris Mini Cooper S, at 6,500 rpm and at 3,500 rpm 1275 The largest A-series engine displaced . It used the bore from the Mini Cooper S versions but the stroke from the plain Mini Cooper. It was produced from 1964 until 1980, when it was replaced by an A-Plus version. The bore size was around the maximum possible in the block, with very little separation between the middle cylinders, which often contributed to head gasket failures. A-Plus versions British Leyland was keen to update the old A-series design in the 1970s. However, attempts at replacement, including an aborted early-70s British Leyland 'K engine' (unrelated to the later Rover K series) and an OHC version of the A series, ended in failure. During the development of what was to become the Austin Metro, engineers tested the A series against its more modern rivals and found that it still offered competitive (or even class-leading) fuel economy and torque for its size. While in the 1970s the A series had begun to seem dated against a new generation of high-revving overhead cam engines, by the end of the decade a new emphasis on good economy and high torque outputs at low speeds meant that the A series's inherent design was still well up to market demands. Given this, and the lack of funds to develop an all-new power unit, it was decided to upgrade the A-series unit at a cost of £30 million. The result was the 'A-Plus' Series of engines. Available in , the A-Plus had stronger engine blocks and cranks, lighter pistons and improved piston rings, Spring loaded tensioner units for the timing chain and other detail changes to increase the service interval of the engine (from ). More modern SU Carburettors and revised manifold designs allowed for small improvements in power without any decrease in torque or fuel economy. Many of the improvements learnt from the Cooper-tuned units were also incorporated, with A-Plus engines having a generally higher standard of metallurgy on all units, where