The MGA was powered by variants of the BMC B-series four-cylinder engine throughout its production life, with three pushrod versions spanning the 1500, 1600, and 1600 Mk II, and the entirely separate twin overhead camshaft engine unique to the Twin Cam model. Understanding which engine is fitted, and crucially, which engine prefix it carries, is the foundation of all parts selection for the MGA.
Pushrod Engine Development
The original MGA 1500 engine was the 1,489cc type BP15GB (later designated 15GB-U-H, where U indicated centre gearchange and H stood for high compression). The bore was 73.025mm with a stroke of 88.9mm, producing approximately 68 bhp at launch (later uprated to 72 bhp) at a compression ratio of 8.3:1.
In January 1959, the revised engine type 15GD-U-H was introduced from chassis number 61504, bringing an important change: the starter motor position was raised, which necessitated a new gearbox mounting plate, a modified gearbox tunnel, a revised right-hand toeboard, and altered carpet. This means that 15GB and 15GD engines are not a straightforward swap without also addressing these surrounding components. The MGA 1600 followed in May 1959 with the engine bored out to 75.39mm for a capacity of 1,588cc, designated 16GA-U-H. This engine shared its bore and stroke dimensions with the Twin Cam engine but was otherwise a conventional pushrod unit producing approximately 79.5 bhp.
The 1600 Mk II, introduced in April 1961, saw the bore further increased to 76.2mm for a capacity of 1,622cc, designated 16GC-U-H at a higher compression ratio of 8.9:1 (or 16GC-U-L at 8.3:1 for low-compression markets). The Mk II engine produced approximately 90 bhp.
Engine Identification
The quickest visual identification is the casting legend on the left-hand side of the crankcase at the front: 15GB and 15GD engines have '1500' cast into the block, 16GA engines have '1600', and 16GC engines have '1622'. The cylinder head also carries a cast number at the rear, '15' on both the 1,489cc and 1,588cc heads, changing to '16' only on the 1,622cc Mk II head. The engine number plate, rivetted to the top of the block on the right-hand side between cylinders two and three, carries the full engine prefix and number which determines the correct specification for all associated components. All MGA pushrod engines were finished in a dark shade of red or maroon, applied to the block, head, sump, rocker cover, and all external engine components.
Engine mounting brackets were painted black. The rocker cover carried a bayonet-type oil filler cap towards the front, finished in cadmium plating and embossed with the brand names of eight recommended lubricants.
The Twin Cam Engine
The Twin Cam engine (type 16GB-U) was a fundamentally different unit sharing only its basic block architecture and bore/stroke dimensions with the pushrod 1600. The aluminium alloy cylinder head carried twin overhead camshafts driven by a duplex roller chain from a half-speed layshaft, with valves set at an included angle of 80 degrees and operated through inverted bucket-type tappets with shim adjustment. With a compression ratio of 9.9:1, it produced approximately 108 bhp, a remarkable output for a 1,588cc engine in 1958. The Twin Cam engine underwent significant modifications during its short production run.
From engine number 2223, the distributor was changed to a type without vacuum advance to prevent over-advancing and consequent piston failure. From engine number 2251, lower-compression pistons at 8.3:1 were introduced, reducing power to approximately 100 bhp but greatly improving reliability. Visually, the Twin Cam is distinguished by its aluminium camshaft covers with cast MG badges picked out in red, and a screw-in oil filler cap on the left-hand cam cover.
Exchange Engines and Cross-Compatibility
Because the B-series engine was used across a wide range of BMC vehicles, it is not uncommon to find MGA cars fitted with non-original engines. BMC supplied reconditioned 'Gold Seal' replacement engines with distinctive prefixes, but many cars have since received engines from MG Magnettes, Austin Cambridges, or early MGBs. Most B-series engines are a relatively straightforward fit in an MGA, with the notable exception of those from the MG Magnette ZA/ZB, Wolseley 15/50, and certain light commercial vehicles which have very different sumps and oil pump pick-up arrangements. The most common non-original swap encountered is an early MGB three-bearing engine with prefixes starting 18G or 18GA.
Always verify the engine number plate before ordering parts, the prefix determines which cylinder head, water pump, manifolds, carburettors, rocker assembly, timing components, oil filter arrangement, and many other items are correct for the engine fitted.
Ordering Considerations
The engine prefix is the single most important reference when selecting parts.