The upper engine components of the MGB cover the valve train, timing drive, and the external covers that seal the top and front of the engine. Many of these components changed across the engine range, and correct identification of the engine series is essential when ordering replacements.
Rocker Cover & Oil Filler
The rocker cover sits on top of the cylinder head and is secured by two studs or nuts depending on the engine type. On 18G, 18GA, and 18GB engines the cover is held by plain nuts, while 18GD, 18GG, and 18V engines use a nut-and-stud arrangement. Chrome plated rocker cover nuts and alloy rocker covers are available as cosmetic upgrades for owners looking to improve the engine bay appearance. The rocker cover gasket should be renewed whenever the cover is removed, a leaking rocker cover is one of the most common oil leaks on the B-series engine and is almost always caused by a hardened or incorrectly seated gasket.
The oil filler cap on the rocker cover changed from a vented metal type embossed with lubricant recommendations on early 18G engines to a sealed plastic type with a built-in vent from the 18GA onwards. Both black and red plastic replacement caps are available.
Tappet Chest Covers
The tappet chest covers, front and rear, seal the sides of the cylinder block where the cam followers operate. The covers, gaskets, and mounting hardware differ between earlier engines (18G, 18GA, 18GB, and 18GD up to a mid-production change point on the 18GG) and later engines (late 18GG and all 18V types).
The later covers may be fitted to earlier engines with little modification, except in the case of open circuit breathing 18G engines which require the unique swan-neck front cover. The gasket thickness also differs, earlier covers use a 1/8-inch gasket, while later covers use a thicker 1/4-inch gasket available in either rubber or cork. The timing cover seals the front of the engine and houses the crankshaft front oil seal. Three different timing covers were used, one for 18G-series engines, one for 18V chrome bumper engines, and a third for 18V rubber bumper engines.
The timing cover gasket and oil seal should be renewed whenever the cover is removed. A jointing compound such as Wellseal is recommended on the mating surfaces for a reliable seal.
Timing Chain & Gears
The timing chain drives the camshaft from the crankshaft. Early engines used a duplex (double-row) chain and gear set, which was changed to a simplex (single-row) chain on later 18V engines. A timing chain tensioner is fitted to maintain correct chain tension throughout its service life, a worn or rattling timing chain that has stretched beyond the tensioner’s range of adjustment should be replaced promptly, as a jumped chain will disturb the valve timing and can cause valve-to-piston contact on engines with tight clearances. For performance applications, a duplex vernier timing gear set is available.
This allows the camshaft timing to be set in increments of one degree rather than to the nearest tooth on the gear, enabling precise optimisation of valve timing for a specific engine build. The duplex chain also provides greater strength and wear resistance than the simplex type.
Camshaft & Followers
The camshaft is available in mild and fast road profiles for 3-main bearing, 5-main bearing, and 18V engine types, supplied as part of a cam kit that includes valve springs, cam followers, cam lube, and a static timing disc. Cam lube must be applied to both the cam lobes and the followers during assembly to guard against scoring during initial start-up of a rebuilt engine. Individual cam followers and pushrods are also available separately, noting that the pushrod and cam follower specifications differ between 18G-series and 18V-series engines. Camshaft bearings, the camshaft thrust plate, and the engine front mounting plate complete the upper engine assembly.
The mounting plate differs between chrome bumper and rubber bumper applications. The MGOC Spares range covers rocker covers, tappet chest covers, timing covers, timing chains, tensioners, vernier timing gears, camshafts, cam followers, pushrods, gaskets, oil seals, and all associated upper engine components for every MGB engine type across the full production run.