A Short History and Description of the BMC B-Series Engine

Details

Written by B.S. Fine

When the Nuffield companies joined long-time rival Austin to form the British Motor Corporation in 1952, the combined group had new opportunities to share research and developments. One happy result of this was that when the MG Z Magnette was launched at the October 1953 Motor Show it was powered by a new 1489cc engine unit developed in the Austin Design Office at Longbridge and not by the Nuffield XPAG engine that continued to be used in the Wolseley 4/44. Over the years, this new engine, to be known as the ‘B’ Series, proved to be strong, reliable, tolerant of poor maintenance and readily adaptable to modification and tuning. It was designed to have a 5-year production life, so the fact that it ceased production 27 years later in 1980 as a 1800cc unit doing service in the Sherpa van testifies to its commercial and technical success.

 The ‘B’ Series engine and its smaller sibling the ‘A’ Series were both developments of an original 1200cc unit used in the pre-war Austin 10/4. It had been fitted to the A40 Devon and Dorset models in 1947. Further development at the ADO took place in 1953 leading to the addition of a 1489cc unit for the A40 Somerset.

 The standard 1489cc engine has a bore of 73.025mm and a stroke of 89mm. When the 1200cc engine was developed to provide the larger capacity, the cylinder centres were spaced wider apart, leading to changes in the crankshaft and its bearing journals. There are three main bearings which carry steel-backed shells coated with softer white metal on the bearing face (later improved to lead-indium). The big ends, which also carry bearing shells, are split diagonally to reduce their width so that piston and connecting rod can be withdrawn upward through the cylinder. The small ends are split so that the gudgeon pin can be clamped in position with a pinch-bolt but this makes the top of the connecting rod both heavy and weak. In the later 1800cc development of the engine the pinch-bolt has disappeared and the pin is located by circlips.

 The single camshaft is mounted low down on the left side of the engine and operates the valves via push-rods and rockers. It is chain-driven off the crankshaft with later versions being tensioned by a self-adjusting tensioner that presses on the chain. The cam followers ride the broad cam lobes slightly off-centre so that as the followers are lifted they also rotate in their housings, thereby spreading wear over their whole base rather than having it in an unwelcome single “stripe”. The distributor and the oil pump are both mounted low and driven by pinion shafts off the camshaft. When the engine was used later in the MGA the shaft also drove the tachometer via a pinion at its rear end. In some applications of this engine the camshaft also drives a mechanical fuel pump and the aperture in the block where this would fit is closed in the Magnette by a simple steel blanking plate. The big ends and balance counter-weights on the crankshaft spin very close to the camshaft, which limits development of the engine because the stroke cannot be lengthened.

 The electrics on the engine are intentionally separated from the fuel and exhaust on opposite sides of the block but this leads to the perceived weakness that the fuel and the exhaust gases enter and exit the engine on the same side, meaning that the valves are side by side in line along the engine and gas-flow is not smooth. This weakness is compounded by  the “Siamese” design of inlet and exhaust ports, which also make engine breathing less than perfect. The carburettor delivers fuel mixture through only two induction ports, which split to serve inlet valves set side by side in neighbouring pairs of cylinders. The exhaust exits via three ports, the centre Siamese one serving the two middle cylinders and creating a hot spot. This can cause running on and head gasket problems. The heart-shaped combustion chamber was designed by Harry Weslake in a consultancy role and was intended to improve gas flow and combustion. The development of an aluminium cross-flow head later by independent engineer Vic Derrington was aimed at eliminating this weakness completely.

The early Magnette engines had a by-pass oil filter that allowed some of the oil to recirculate unfiltered but, when rapid bearing wear was identified as a designed-in problem,  this was later replaced by a full-flow filter linked to the system by a pipe off the main oil gallery at the right hand rear of the engine. Oil pressure is controlled by a spring-loaded by-pass valve that opens at 50psi to protect the engine from excessive pressure when the engine is operating at high speed and when the viscosity of the oil is high before the engine warms up. Engines often suffer minor but irritating oil leaks from the front timing cover and the rear of the crankshaft. At the front, a felt ring was used where the front of the crankshaft passes through the timing cover behind the starter dog. This ring tends to shrink with age and the seal is lost. At the rear, oil is retained only by a reverse scroll and tends to find its way out of the engine, into the bell-housing and onto the ground via the bell-housing drain hole.

 Later developments of this engine included boring it out to as much as 2 litres cubic capacity and adding two more main bearings for more strength and crankshaft stability. Although the MGB was introduced with a 1800cc 3 main bearing engine similar in most respect to the MGA 1622cc unit, this was soon discontinued in favour of a 5 main bearing unit, to overcome the perceived weakness of the bottom-end when performance tuning was required. In spite of the design changes over its production life, any B-series cylinder head will fit any B-series block. There are advantages in fitting an early engine with a later head with improved porting and larger valves, but with heads from 1800cc engines, care is needed because the valves overlap the cylinder bore edges and the block face will need relieving to avoid a disasterous impact. Care is also needed because a different volume combustion chamber will change the compression ratio.

 The general similarity of the 1800cc unit to the original 1500cc predecessor and the numbers of  second-hand units available make it a suitable upgrade option for the Z Magnette. However, towards the end of its production run,  the MGB was blighted by emission control equipment needed to satisfy the statutory requirements in the MG factory’s valuable USA market. This actually reduced the engine’s power output, so the Z Magnette owner is well advised to choose one of the earlier 5-bearing engines without the extra “plumbing”. Although it is convenient to find MGB engines, very similar units were fitted to other  cars including the Morris Marina 1800 TC, which may also yield the parts required. For more guidance on fitting the 1800cc engine to the Z Magnette, please refer to the Modifications section.