The lower engine components of the MGB encompass the crankshaft, main and big end bearings, pistons and rings, connecting rods, sump, oil pump, flywheel, and the associated gaskets and seals that form the foundation of the B-series engine. Several of these components changed across the engine range, and the distinction between three main bearing and five main bearing engines is fundamental to correct parts selection.
The first MGB engines, the 18G and 18GA series from 1962 to 1964, used a crankshaft with three main bearings. From October 1964, the 18GB series introduced the five main bearing crankshaft, a substantial redesign that required a modified cylinder block and provided significantly improved bottom-end rigidity and refinement. The five-bearing crankshaft was retained for all subsequent MGB engines through to the end of production in 1980. Several different crankshaft types are documented across the 18GB engine range, but all are interchangeable.
The main bearing and big end bearing sets differ between three and five main bearing engines and are available in standard and a range of undersize dimensions to suit reground crankshafts. Crankshaft thrust washers control end float and are available in standard and oversize thicknesses.
Pistons & Connecting Rods
Pistons changed several times during production. Three main bearing 18G and 18GA engines used a pinch-bolt type gudgeon pin arrangement, where the gudgeon pin was clamped in a split con rod small end. From the 18GB onwards, the gudgeon pin became fully floating with circlips for location.
Later 18V engines adopted a press-fit gudgeon pin into the connecting rod, dispensing with the small end bushing and circlips of the earlier design. Piston ring sets differ between the earlier four-ring type and the later three-ring type used on 18V pistons. All pistons are available in standard and a range of oversizes for rebored blocks. Connecting rods are specific to the engine series and underwent changes to their profile, later 18V rods adopted a tapered shape with horizontally split big ends.
Con rod bolts, nuts, and tab washers should always be renewed when rebuilding the bottom end, as these are high-stress fasteners subject to stretch. The sump is a pressed steel item that bolts to the bottom of the cylinder block and was modified at various points during production, though the changes are mostly of a minor nature and later sumps are generally interchangeable with earlier ones. The sump gasket should always be renewed when the sump is removed. The sump drain plug and its sealing washer are accessible from underneath the car and should be checked for condition at every oil change.
The oil pump is driven by gear from the camshaft and sits within the sump. The pump type differs between three main bearing and five main bearing engines. A repair kit is available for the five main bearing pump. The oil strainer, pressure relief valve, and associated spring and packing piece are also available individually.
The oil pressure relief valve controls the maximum oil pressure in the system and should be inspected during any bottom-end rebuild.
Flywheel & Rear Seal
The flywheel is bolted to the rear of the crankshaft and differs across the production range. Three main bearing engines used one type, early five main bearing engines with inertia starter motors and 3-synchro gearboxes used another, and later five main bearing engines with pre-engaged starter motors and 4-synchro gearboxes used a third. The ring gear on the flywheel differs between inertia and pre-engaged starter motor types.
Cars with automatic gearboxes used a drive plate in place of the flywheel. Reconditioned exchange flywheels are available for all types. The rear crankshaft oil seal prevents oil from escaping at the junction between the crankshaft and the engine backplate, and is fitted on five main bearing engines only. The engine backplate itself differs between three main bearing, early five main bearing with 3-synchro gearbox, and later five main bearing with 4-synchro or automatic gearbox applications.
Backplate gaskets, upper and lower, and the associated screws, lock tabs, and blanking plugs are all available individually. The dipstick and its tube changed during production, with several types used across the 18G, early 18V, and later 18V engine ranges. An alloy dipstick is available as an alternative for 18V engines from 1971 onwards. The MGOC Spares range covers crankshafts, bearing sets, pistons and rings, connecting rods, sumps, oil pumps, flywheels, ring gears, rear oil seals, engine backplates, gaskets, and all associated lower engine components for every MGB engine type across the full production run.