The MGB inlet manifold sits on the left-hand side of the cylinder head and carries the twin SU carburettors. Two fundamentally different types of inlet manifold were used across production for cars with twin carburettors, and they are distinguished by their flange thickness where the manifold meets the cylinder head. The thick-flange inlet manifold was used on all 18G-series engines from the start of production in 1962 through to 1971. Within this period there were several variations, mostly owing to changes in the number and position of adaptor plugs fitted to the balance pipe that connects the two carburettor throats.
These adaptors accommodated ancillaries such as the overdrive vacuum switch on 3-synchro gearbox cars, the brake servo connection, and the vacuum advance pipe. Despite these variations, the thick-flange manifolds are broadly interchangeable within the 18G engine family. The thin-flange inlet manifold was introduced with the 18V-series engines from 1971 and continued to the end of production in 1980. This manifold incorporated revised adaptor positions for the vacuum advance pipe and brake servo connection.
The manifold gasket is common to both thick and thin-flange types and should always be renewed when removing or refitting the manifold.
Carburettor Mounting & Heatshield
The carburettor mounting arrangement differs between carburettor types. Cars fitted with the smaller 1½-inch HS4 or HIF4 carburettors use four manifold-to-carburettor studs with spacer blocks between the manifold and carburettor flanges.
Cars fitted with the larger 1¾-inch HS6 carburettor conversion use eight studs and a different spacer block. The carburettor-to-manifold gaskets also differ between the two carburettor sizes. When refitting or changing carburettors, it is good practice to renew the manifold gaskets, spacer blocks, and all associated studs and nuts at the same time. A heatshield is fitted between the exhaust manifold and the inlet manifold side of the engine to reduce the transfer of exhaust heat to the carburettors and fuel system.
The correct heatshield differs depending on which carburettor type is fitted and, on HIF4 cars, whether the car was built before or after September 1976. Cars fitted with the HS6 carburettor conversion require a stainless steel heatshield pair specific to the larger carburettor installation. A deteriorated or missing heatshield can contribute to fuel vaporisation and heat soak, particularly in traffic or on warm days, leading to rough running, difficult hot starting, and fuel percolation from the carburettor overflow pipes. The MGOC Spares range covers both thick and thin-flange inlet manifolds, manifold gaskets, carburettor mounting studs, spacer blocks, heatshields, anti run-on valves, and all associated fixings to cover every MGB carburettor configuration across the full production run.