The MGB was powered by the BMC 1798cc B-series four-cylinder engine throughout its eighteen-year production run from 1962 to 1980. While the fundamental architecture remained the same, an overhead valve design with pushrod-operated valves, a cast iron block, and a pressed steel sump, the engine was continuously developed and updated, with each significant revision marked by a new engine number prefix series. Understanding the engine series fitted to the car is essential when ordering a replacement engine or any associated components.
Engine Development
The first MGB engine in 1962 was the 18G series, featuring a crankshaft with three main bearings. This was followed in early 1964 by the 18GA, which introduced the closed circuit breathing system but retained the three-bearing crankshaft. In October 1964, the landmark 18GB series was introduced with a new five main bearing crankshaft, a substantial redesign requiring a modified cylinder block, which significantly improved bottom-end strength and refinement. The five-bearing crankshaft would remain the foundation of all subsequent MGB engines.
From 1967, the pace of change accelerated as emissions regulations required specially adapted engines for different markets. The 18GD and its North American counterpart the 18GF introduced the fully synchronised gearbox and alternator. The 18GG and 18GH series followed in 1968 with revised crankcase ventilation.
From 1971, the 18V series engines brought further changes including revised carburettor specifications, a commonised water pump, and a switch from the duplex to simplex timing chain on later engines. The 18V series continued through chrome bumper and rubber bumper production to the end in 1980, with numerous sub-variants developed for different markets and emissions requirements.
Engine Identification
Between 1962 and 1971, engines were painted dark red or maroon. From 1971 with the introduction of the 18V series, engines were painted black. The 1798cc B-series can be identified by the legend ‘1800’ cast in the crankcase on the left-hand side towards the front.
Reconditioned & Performance Engines
It is not uncommon to find a non-original engine in an MGB. Factory reconditioned replacement engines were supplied under the Gold Seal label and can be identified by their special engine number prefixes. Engines from other vehicles using the same 1800cc B-series unit, such as the Austin 1800 or Morris Marina, are also sometimes found, though these may differ in ancillaries and require adaptation.
When ordering a replacement or reconditioned engine, the engine prefix and number should be checked against the documented specifications to ensure the correct type is supplied. Reconditioned exchange engines are available across the range of MGB applications, built to standard or performance specifications with unleaded-tolerant cylinder heads. Performance engines are offered in a range of capacities and tuning stages, from mild road-specification builds through to high-torque and Stage II configurations with ported and polished heads, performance camshafts, and larger capacity options. The most heavily modified builds may require complementary upgrades to the cooling system, carburettors, and exhaust to realise their full potential, and owners of larger-capacity engines should note that adequate cooling fan provision is important to preserve the engine warranty.
The MGOC Spares range includes reconditioned exchange engines and performance engines to suit every MGB application, supported by the full range of individual engine components, gaskets, and ancillaries covered throughout the Engine section of this catalogue.