The inlet manifold arrangement on an MGA is a direct reflection of its carburettor type and differs sharply between pushrod cars and the Twin Cam. Pushrod 1500, 1600 and 1600 Mk II engines use a single cast-iron inlet manifold carrying both H.4 carburettors via a crossover balance pipe; the Twin Cam uses a pair of separate inlet manifolds feeding its HD.6 carburettors. Both arrangements include a heat shield between the carburettor bodies and the adjacent exhaust manifold. Ordering for this node therefore begins with the engine type and follows through into the matching balance pipe, heat shield and fastening hardware.
Pushrod inlet manifold, 1500, 1600 and 1600 Mk II
All pushrod-engined MGAs use a single inlet manifold bolted to the right-hand side of the cylinder head, carrying both H.4 carburettors and the crossover balance pipe that links them. The manifold is finished in engine colour during a correct restoration, the same dark red or maroon used on the block, head, sump and rocker cover. The balance pipe itself is also painted engine colour and sits between the two carburettor bodies at the top of the manifold assembly.
Replacement manifolds, balance pipes, inlet-side studs, manifold-to-head gaskets and the brass nuts that secure the manifold are all available as service items. When replacing the manifold, the matching exhaust manifold gaskets and studs should generally be inspected at the same time since the inlet and exhaust manifolds share common fixings on the right-hand side of the cylinder head.
Twin Cam inlet manifolds
The Twin Cam uses a quite different inlet arrangement: paired cast aluminium inlet manifolds, one per HD.6 carburettor, bolted to the Twin Cam cylinder head. Both manifolds are finished in red as part of the Twin Cam's red-and-aluminium engine-bay colour scheme, against which the natural aluminium camshaft covers stand out. Twin Cam manifolds are not interchangeable with pushrod items, either in port matching, in fixing pattern or in carburettor flange dimension, HD.6 carburettors are 1¾ inch and require the larger manifold flange.
Replacement manifolds, their studs, gaskets and the associated small fixings are ordered specifically to the Twin Cam specification.
Carburettor balance pipe
On pushrod cars the balance pipe is a distinct, named item, a crossover between the two H.4 carburettors that equalises manifold vacuum between the front and rear float chambers. It is finished in engine colour.
Replacement balance pipes, their union fittings and the short hose sections at each end are available as service items. On the Twin Cam, the equivalent balance function is integral to the paired manifold castings rather than supplied as a separate pipe, so Twin Cam orders do not include a balance pipe as a service item.
Heat shield
A heat shield sits between the carburettor bodies and the exhaust manifold on all MGAs, protecting the float chambers from radiated exhaust heat and preventing fuel vaporisation in the carburettor bowls during slow running. On pushrod cars the heat shield is finished in engine colour (red/maroon) to match the rest of the engine's painted hardware. On the Twin Cam it is likewise red, continuing the red-block theme alongside the red inlet manifolds and against the natural-aluminium camshaft covers.
Replacement heat shields, their mounting stays, insulating spacers and the fixings that secure them to the inlet manifold studs are available as service items. When refitting, the insulating spacers should be renewed rather than reused, since compressed fibre spacers lose effectiveness after heat-cycling and contribute to hot-start fuel vaporisation if they have hardened in service.
Ordering considerations
The engine prefix determines which manifold set is required. Pushrod cars (15GB, 15GD, 16GA, 16GC) use the single H.4-pattern manifold and balance pipe; the Twin Cam uses the paired HD.6-pattern manifolds without a separate balance pipe. Manifold-to-head gaskets should always be renewed when the manifold is disturbed, as the gasket compresses on first fitment and will not reliably reseal if reused.
For cars that have been re-engined with an MGB unit, an MGB HS-pattern manifold will be fitted rather than any MGA item, and those service parts are not in the MGA catalogue.