The lower engine encompasses the crankshaft, connecting rods, pistons, main and big-end bearings, timing chain and gears, camshaft, cam followers (tappets), oil pump, sump, and all associated gaskets and seals. These are the components that define the engine's capacity, compression ratio, and fundamental mechanical condition. On the MGA, all three pushrod variants share the same basic B-series architecture, but the bore size, and therefore the piston, ring, and gasket specifications, differ between the 1489cc 1500, 1588cc 1600, and 1622cc Mk II.
Crankshaft and Bearings
All MGA pushrod engines use a three-bearing crankshaft, three main bearings supporting the crankshaft in the block, with four connecting rods running on the crankshaft's big-end journals. Main bearing and big-end bearing shells are available in standard and undersize sets for crankshafts that have been reground.
When ordering bearings, the required size must be stated, standard, -0.010", -0.020", -0.030", or -0.040" undersize. Thrust washers control crankshaft end float (the fore-and-aft movement of the crankshaft in the block) and are available in standard and oversize thicknesses.
Pistons and Rings
Pistons are available in standard bore and oversizes corresponding to the standard rebore increments: +0.010", +0.020", +0.030", and +0.040" over standard. The bore size differs between variants, 73.025mm for the 1500, 75.39mm for the 1600, and 76.2mm for the Mk II, so the correct variant must be specified. Pistons are supplied as a set of four for all cylinders. Standard-compression pistons (8.3:1 on the 1500 and 1600) are of the 5-ring type; higher-compression 9:1 pistons are available as a 3-ring type where required.
The Mk II's higher compression ratio (8.9:1 standard versus 8.3:1 on the 1500 and 1600) is achieved through a revised piston crown profile, and the Mk II's larger bore also affects the gudgeon pin diameter.
Timing Chain, Gears and Tensioner
The timing drive uses a duplex (double-row) chain running between the crankshaft and camshaft gears. A spring-loaded timing chain tensioner, fitted to all MGA engines beyond the very earliest 1500 production, maintains chain tension via a rubbing block that bears on the slack side of the chain. A worn timing chain or tensioner rubbing block manifests as a characteristic rattle at idle, particularly noticeable on start-up, and introduces ignition timing scatter that no amount of distributor adjustment can correct. The timing chain, crankshaft gear, camshaft gear, and tensioner rubbing block should all be inspected together during any engine rebuild and replaced as a set where any significant wear is present. A vernier duplex timing gear set is available as a performance upgrade, allowing precise adjustment of camshaft timing relative to the crankshaft and offering greater accuracy when setting the valve timing of a non-standard camshaft.
Camshaft and Followers
The camshaft runs in bearings pressed into the block and is driven by the timing chain. It operates the eight cylindrical cam followers which in turn push the pushrods that activate the rocker arms above. A worn camshaft produces low compression readings, poor engine performance, and a characteristic clatter from the valve train. On MGA engines using modern low-ZDDP oils, the camshaft lobes and followers are particularly vulnerable to accelerated wear, classic engine oils formulated with adequate zinc and phosphorus content should be used, particularly during break-in after a rebuild.
Oil Pump
The oil pump is driven by a vertical drive spindle taking its drive from a skew gear on the camshaft, and it delivers oil under pressure through the engine's internal galleries. The distributor drive is taken from the same shaft. Oil pump condition directly affects oil pressure, a worn pump will show low pressure at idle and under load. The oil pressure relief valve is housed in the block and controls the maximum system pressure. The oil pump originally fitted to the earliest 1500 engines (before chassis number 15GB46342) is no longer available as a direct replacement, but the later service pump is a drop-in substitute provided the correct strainer and mounting studs are also fitted.
Sump
The sump is a pressed steel unit bolted to the bottom of the block. The sump gasket must match the block's lower flange width. The sump drain plug is a magnetic type that captures ferrous particles from the oil, inspecting the drain plug during an oil change gives useful information about internal wear.
Ordering Notes
When ordering lower engine components, the engine prefix (15GB, 15GD, 16GA, or 16GC) is essential as it determines the bore size, bearing specification, and gasket requirements. For complete rebuilds, a conversion gasket set provides all the lower engine gaskets. The timing cover oil seal and rear crankshaft seal are purchased separately.