BMC A-Series engine
British motor corporation
Mini 850 A series engine
Austin Motor Company's small straight-4 automobile engine, the A-Series, is one of the most common in the world. Launched in 1951 with the Austin A30, production lasted until 2000 in the Mini. It used a cast-iron block and cylinder head, and a steel crankshaft with 3 main bearings. The camshaft ran in the cylinder block, driven by a single-row chain for most applications, and with tappets sliding in the block, accessible through pressed steel side covers for most applications, and with overhead valves operated through rockers. The cylinder head for the overhead-valve version of the Austin series A engine was designed by Harry Weslake – a cylinder head specialist famed for his involvement in SS (Jaguar) engines and several F1 title winning engines.
The A-Series design was licensed by Nissan of Japan, along with other Austin designs. That company quickly began modifying the A-Series as the Nissan A engine, and it became the basis for many of their following engines.
All engines had a cast iron head and block, and two valves per cylinder in an OHV configuration. Engines were available in Diesel in the BMC tractor.
All A-series engines up until mid-1970 were painted in British Standard (381c) 223 Middle Bronze Green. : Source Austin Morris (BL) internal documents archives. This does not include overseas production models such as Australian manufacture.
The original A-Series engine displaced just 803 cc and was used in the A30 and Morris Minor. It had an undersquare 58 mm (2.3 in) bore and 76.2 mm (3.00 in) stroke. This engine was produced from 1952–56.
1956 saw a displacement increase, to 948 cc. This was accomplished by boring the block out to 62.9 mm (2.48 in) while retaining the original 76.2 mm (3.00 in) stroke. It was produced until 1964.
A diesel version of the 948 cc A-Series engine (producing 16 hp (12 kW) at 2,500 rpm and 38 lb·ft (52 N·m) torque at 1,750 rpm) was produced for the BMC Mini Tractor. It was developed with the help of Ricardo Consulting Engineers. This engine has dry liners. The block is almost identical to the petrol engine. the oil pump has been removed from the camshaft and is driven by an extension to what would have been the distributor drive. A petrol version of this modified engine was 'reverse-engineered' for use in the Mini Tractor whilst retaining parts commonality with the diesel variant, rather than using a standard petrol A-Series unit. The diesel A-Series was also sold as a marine engine under the BMC name alongside the diesel B-Series engines.
The 62.9 mm (2.48 in) bore was retained for 1959s 848 cc Mini version. This displacement was reached by dropping the stroke to 68.26 mm (2.687 in). This engine was produced through to 1980 for the Mini, when the 998 A-Plus version supplanted it.
The one-off 997 cc version for the Mini Cooper used a smaller 62.43 mm (2.458 in) bore and longer 81.28 mm (3.200 in) stroke. It was produced from 1961–1964.
The Mini also got a 998 cc version. This was similar to the 948 in that it had the same 76.2 mm (3.00 in) stroke but was bored out slightly to 64.58 mm (2.543 in). It was produced from 1962–92.
The 1.1 L (1098 cc) version was produced for the larger BMC saloons. It was a stroked (to 83.72 mm (3.296 in)) version of the 998 previously used in the Riley Elf and Wolseley Hornet. It was produced from 1962–80.
The 1071 cc version was another one-off, this time for the Mini Cooper S. It used a new 70.6 mm (2.78 in) bore size and the 68.26 mm (2.687 in) stroke from the 848. It was only produced in 1963–1964. Paired with the even rarer 970 cc version, below, it became that rarest of things: an oversquare A-series engine.
The Mini Cooper S next moved on to a 970 cc version. It had the same 70.6 mm (2.78 in) bore as the 1071 cc Cooper S but used a shorter 61.91 mm (2.437 in) stroke. It was produced from 1964–1965.
The largest A-Series engine displaced 1.3 L (1275 cc). It used the 70.6 mm (2.78 in) bore from the Mini Cooper S versions but the 81.28 mm (3.200 in) stroke from the plain Mini Cooper. It was produced from 1964 until 1980, when it was replaced by an A-Plus version.
British Leyland was keen to update the old A-Series design in the 1970s. However, attempts at replacement, including an aborted early-70s Rover K engine and an OHC version of the A-Series, ended in failure. During the development of what would 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' 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 998cc and 1275cc, the A-Plus had stronger engine blocks and cranks, lighter pistons and improved piston rings, hydraulic 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.
The new engines received distinctive 'A+' branding on their rocker covers and the blocks and heads were colour-coded for the different capacities: yellow for 998 cc and red for 1275 cc engines.
The A-Plus version of the 998 cc motor was produced from 1980–92.
The big 1.3 L (1275 cc) engine was also given the "A-Plus" treatment. This lasted from 1980–2000, making it the last of the A-Series line.
Turbo versions lasted from 1983–90.
A special "twin-port injection" version of the 1.3 L (1275 cc) engine was developed by Rover engineer, Mike Theaker. It was the last A-Series variant, produced from 1997–2000.
MG Metro Turbo engine
BMC A-Series engine Technical details and specifications (1951-2000)
848cc - 998 cc Engines 8MB/8AM/85H/99H (1959-1974)
No. of cylinders 4/OHV
Capacity 848 cc and 998 cc
Firing order 1-3-4-2
Compression ratio 8.3:1
Suitable for unleaded petrol No
Fuel system 848cc Make SU Type HS2 Carb-VV
Fuel system 998cc Make SU Type HS2/HS4 Carb-VV
Ignition coil Make Lucas Type LA12
Distributor Make Lucas No. 25D4/45D4
Distributor Contact breaker gap 0.4mm
Distributor Dwell angle 848cc 57-63° 46-56 (%) 998cc 57-63° (63-70%)
Fuel pump pressure 0.17-0.21 bar
Starter motor Make Lucas Type M35G/M35J
Maximum cranking amps 97-119 A
Voltage regulator Make Lucas RB 106/2
Regulated voltage 13.9-14.3V
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BMC A-Series engine Practical Instructions and Service Guide (1951-2000)
848cc - 998 cc Engines 8MB/8AM/85H/99H (1959-1974)
Spark plugs Original equipment Champion Type RN9YCC
Spark plugs Electrode gap 0.8mm
Spark plugs Tighten 25-35 Nm
Valve clearance -INLET 0.30mm check cold
Valve clearance -EXHAUST 0.30mm check cold
Drive belt size - alternator 9,5x825 mm
Drive belt tension - alternator 25mm
Engine oil grade - normal/moderate climate 10W/30 SAE
Engine oil change 5 litres
Cooling system 3.5 litres
Road wheels Tighten 50-57 Nm
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