The MGB exhaust manifold bolts directly to the cylinder head using studs through a combined inlet and exhaust manifold gasket. The manifold specification changed between the 18G-series and 18V-series engines and this affects the studs, gasket, and manifold itself.
OE Manifold & Studs
On 18G, 18GA, 18GB, 18GD, 18GF, 18GG, 18GH, 18GJ, and 18GK engines, all pre-18V engines produced from 1962 to August 1971, the exhaust manifold uses a thick flange at the cylinder head joint. These engines use two different stud lengths: four inner manifold-to-head studs of 5/16 x 1 7/8 UNF and two outer studs of 5/16 x 1 5/8 UNF. On 18V engines from August 1971 to end of production in 1980, the manifold uses a thin flange design, and all six manifold-to-head studs are the same length, 5/16 x 1 5/8 UNF throughout. Selecting the correct stud length is essential: ordering the wrong type will result in insufficient thread engagement or interference with the manifold casting.
The combined inlet and exhaust manifold gasket suits all standard twin-carburettor MGB models. A separate USA specification manifold gasket is also available, which suits the later North American export engines fitted with a single Zenith-Stromberg carburettor on a combined inlet and exhaust manifold, introduced from December 1974 on the 18V-797 and 18V-798 series engines for the 1975 model year. Manifold studs are secured by thick domed washers which provide the correct clamping face against the manifold flange, backed by spring washers. Standard length brass nuts are the original fastener specification, and long brass nuts are also available as an easier-access alternative particularly suited to inner stud positions which can be difficult to reach with standard-length tooling.
A separate set of combination UNC/UNF studs and their own brass nuts are available for the downpipe-to-manifold joint, along with a rubber downpipe seal which fits between the downpipe flange and the manifold outlet. A large exhaust valve (36.6mm) is also listed for engine rebuilds requiring an exhaust valve replacement.
Performance Manifolds
For owners seeking improved performance, two aftermarket manifold options are available. The stainless steel B-series header manifold provides a free-flow four-into-one design for the B-series engine, improving exhaust scavenging and top-end power over the standard cast iron unit. The Double S performance manifold offers a stage-one performance upgrade with improved gas flow characteristics. Both are available.
The MGOC Spares range covers all manifold gaskets, manifold studs, thick washers, spring washers, brass nuts in standard and long formats, downpipe studs, downpipe seal, exhaust valve, and both stainless header and Double S performance manifold options.