The MGA exhaust manifold arrangement is one of the sharpest visible differences between the pushrod cars and the Twin Cam when the bonnet is lifted. Pushrod MGAs run a single three-branch cast iron manifold matched to the B-series cylinder head's three exhaust ports; the Twin Cam runs a pair of separate cast iron manifolds grouped by cylinder pair. Both installations attach to the cylinder head via studs and brass nuts, seal to it through a manifold-to-head gasket, and connect forward to the downpipe (pushrod) or downpipes (Twin Cam) through a flange joint sealed by a second gasket. This node covers all the manifold hardware, castings, gaskets, studs, nuts and downpipe flange fittings, for both engine types.
Pushrod three-branch manifold
All pushrod-engined MGAs, 1500, 1600 and 1600 Mk II, use the same single three-branch cast iron exhaust manifold. The BMC B-series cylinder head carries three exhaust ports: a single outlet for cylinder 1, a central siamesed outlet serving cylinders 2 and 3, and a single outlet for cylinder 4. The three-branch manifold matches this port pattern, with three collector branches merging into a single downpipe connection at the bottom. The manifold is bolted to the left-hand side of the cylinder head using studs screwed into the head casting and brass nuts over the manifold flange.
Brass nuts are used in place of steel at this location because brass-on-steel thread engagement resists galling through repeated heat cycling and allows the nuts to be removed years later without the thread picking up or seizing.
Twin Cam manifolds
The Twin Cam cylinder head carries all four exhaust ports on the left-hand side of the head, grouped to feed two separate cast iron exhaust manifolds rather than a single three-branch casting. The front manifold serves cylinders 1 and 4, the front and rear cylinders of the engine, and the rear manifold serves cylinders 2 and 3, the two middle cylinders. This front/rear cylinder pairing is distinctive to the Twin Cam and reflects the different exhaust gas firing interval grouping compared with the siamesed central pair of the pushrod head. Each manifold carries its own downpipe forward; the two downpipes merge into a single front pipe well forward of the silencer.
Twin Cam manifolds are not interchangeable with pushrod items in any respect, port pattern, flange dimensions, mounting stud positions and downpipe flange are all different.
Manifold-to-head gaskets
The gasket between the manifold and the cylinder head carries the full temperature of the exhaust stream and must seal through repeated heat cycling. A leaking manifold gasket is audible as a ticking or blowing noise from the manifold area that worsens under load and particularly on cold starts before the gasket material has expanded into the joint.
Original-specification gaskets used asbestos-based material typical of the period; modern replacements use multi-layer steel or graphite-composite construction, which offers better long-term performance against modern fuel and operating-temperature profiles. The manifold-to-head gasket should always be renewed when the manifold is disturbed, as the gasket compresses on first fitment and will not reliably reseal if reused.
Studs, nuts and brass hardware
Manifold studs are the service items most commonly encountered on exhaust work. They screw into the cylinder head and carry the full thermal cycling of the manifold over decades of service, with the result that extracting a seized stud for renewal is often a more significant task than the rest of the manifold work combined.
Replacement studs and brass nuts are available as service items; when removing original studs, a stud extractor and penetrating oil applied over several days is the normal approach, and the head-side thread should be cleaned with a tap before new studs are fitted. Where a stud thread in the cylinder head has been damaged on extraction, thread repair inserts are the standard fix, carried out with the head removed or with careful drilling protection to prevent swarf entering the cylinder.
Downpipe flange hardware
The downpipe flange joint between the manifold (or pair of manifolds on the Twin Cam) and the downpipe uses a separate gasket, studs and nuts from the manifold-to-head joint. This joint also experiences full exhaust temperature and cycling, and its hardware, downpipe gasket, flange studs and nuts, should be treated as service items to be renewed whenever the downpipe is disturbed. Pushrod cars have one such joint at the manifold; Twin Cam cars have two, one at each manifold.
Ordering considerations
The primary ordering decision is pushrod versus Twin Cam.