The MG Midget was powered by two fundamentally different engine families. From 1961 to 1974, BMC's A-Series engine was used in three displacements: 948cc, 1098cc, and 1275cc.
From 1974 to 1979, the Triumph 1500cc unit was fitted. Understanding which engine variant is fitted is essential when ordering, as interchangeability between these engines is limited.
948cc and 1098cc
The earliest Midgets (Mk1, GAN1) used the 948cc A-Series with prefix 9CG, producing 46bhp, the same unit fitted to the contemporary Austin-Healey Sprite Mk II. In October 1962, the engine was enlarged to 1098cc under prefix 10CG, with power rising to 56bhp, and the chassis designation advanced to GAN2, the same point at which front disc brakes were introduced. The 10CG retained the 1.75-inch crankshaft main bearing diameter inherited from the 948cc unit. The Mk2 (GAN3, March 1964) brought the revised 10CC prefix, enlarging the main bearing diameter from 1.75 to 2 inches to address crankshaft breakage in the earlier 10CG units, with power rising to 59bhp at 5750rpm on an 8.9:1 compression ratio.
Many internal components are not interchangeable between 10CG and 10CC engines because of this bearing change.
1275cc Engine
The 1275cc A-Series arrived for the 1967 model year (Mk3, GAN4), producing 65bhp at 6000rpm. Key changes over the smaller-bore units included a duplex timing chain, six-bolt flywheel fixing, and a crankshaft-mounted vibration damper doubling as the fan belt pulley.
1275cc Variants
Five 1275cc variants were used in home-market Mk3 Midgets. The 12CC (1967) had a thin lower block flange. The 12CE (from 1968) introduced a thicker flange, the preferred block for rebuilds and competition as it resists distortion under load.
From October 1969, the facelifted GAN5 chassis designation was introduced with BL badges, Rostyle wheels as standard, and a revised grille, though the engine remained the 12CE. The 12V586F (1972), 12V588F (1973), and 12V778F (1974) followed, meeting evolving emissions requirements. The 12V-prefix engines were painted black rather than the earlier MOWOG metallic green.
Cylinder Head
The 1275cc cylinder head differs between vertical-flow and crossflow cooling configurations. The thermostat housing outlet points upward on vertical-flow cars (12CC and early 12CE) and to the left-hand side on crossflow cars.
When ordering heads, gaskets, or thermostat housings, the cooling system layout must be confirmed. Three distinct rocker arm types were fitted (forged, pressed steel, and sintered), each requiring matching adjusting screws, these components must not be mixed.
Oil Filtration
The oil filtration system changed twice during 1275cc production, and the three types are not interchangeable. Bowl-type paper element filters were used on the earliest engines (up to 12CE/Da/H10308). Spin-on canister filters followed from 12CE/Da/H10309 through to 12V778392. A revised filter head and adaptor was introduced at 12V778F393.
A spin-on conversion kit is available for upgrading from the bowl-type assembly.
1500cc Triumph Engine
The Midget 1500 (GAN6, 1974) adopted the Triumph Spitfire's 1493cc OHV engine, designated PE94/J, producing 65bhp at 5500rpm with more torque at lower revs. The Triumph engine shares no components with the A-Series, block, head, sump, timing chain, oil pump, water pump, and all internals are unique. The starter motor mounts on the opposite side, necessitating a revised battery terminal layout. The 1500 uses a single-row timing chain with a separate tensioner, and the rocker gear uses individually numbered arms with distinct pillar arrangements.
Oil Cooler
Oil cooler kits are available for all models. The hose kits differ between vertical-flow, crossflow, and 1500 cooling layouts, so the correct kit must be specified. Engine and gearbox mounting arrangements also differ between the 1275cc and 1500cc. The 1500 introduced a gearbox steady bracket to prevent tunnel contact under load, not fitted to the very earliest 1500 production cars.
Ordering Notes
The most critical information when ordering engine parts is the engine serial number prefix. This determines the exact variant and which components are correct. Pistons and bearings require the oversize to be stated, oil filters depend on the engine serial number range, and cooling system components depend on the radiator layout. For engine rebuilds, a conversion gasket set is used alongside the head gasket set, timing cover and rear crankshaft seals must be purchased separately.