difference between a series and a plus engine
BMC A-series engine 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 Â£30 million. The result was the 'A-Plus' Series of engines. Available in 998cc and 1275cc, 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 6,000 to 12,000 miles). 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 previously only the highest-tuned engines were upgraded in this way. This made the A-Plus engines generally longer-lived than the standard A series, which had a life between major rebuilds of around 80,000 to 100,000 miles in normal service. Studies were made into upgrading the engine to use five main crankshaft bearings but the standard three-bearing crank had proven reliable even in high states of tune and at high engines speeds, so it was not deemed worth the extra funding.