Manifold Seating – How to Fix an Intake/Exhaust Leak
My manifolds are not sealing down very well, I’ve got an intake/exhaust leak. What should I do?
You can check the mating surfaces on a piece of plate glass. If you see any gaps have them planed the minimum necessary. Do the inlet and exhaust separately. Mounting manifolds on a non-crossflow inline head can be frustrating. Take your time and make sure everything is clean before starting. Ensure that the carb, linkage and vacuum hoses are removed or safely out of the way. Plan on this taking at least an hour to do this correctly.
The head should have studs to mount the manifolds – if bolts were installed in the past, chase (clean) the threads and install studs. Install gaskets with a little copper silicone to hold them in place (Clifford makes a nice one-piece inlet/exhaust gasket which is easier to install than the separate type).
If you’re using aftermarket manifolds, the clamps may require grinding to fit. If you can find them, use undersize eccentric locknuts on the studs, otherwise use brass nuts with lockwashers. put the “bowtie” clamps on the inside studs and install nuts loosely. You should be able to hang the exhaust manifold using the outermost studs, then work the intake into place. Definitely use the alignment rings if available. They can be epoxied into the inlet manifold to ensure they don’t slip out of place. You should be able to feel the rings pop into place in the ports. A rubber hammer (or hide mallet as the Brits call them) can be of use here. You can then loosely bolt the manifolds together at the heat riser. Then start tightening all the nuts: start at the middle and work out toward each end, a turn or two at a time, repeated until manifolds are tightly attached to the head. Only then should you tighten the two manifolds together at the heat riser box.
For some reason (age, warping, parts swapping) every one of these stovebolts seems to go together a little bit different in the manifold area – using new fasteners, chasing threads, and pre-fitting everything seems like a lot of trouble but will save time and headaches in the long run. Using the port alignment rings makes the whole job go much easier; if the manifolds are so badly warped that major machining is needed, make sure both manifolds are planed an equal amount, or replace them.
Often the stock manifolds have been planed down many times. This can lead to the manifold not appearing to fit properly, as if it is binding. This is exactly what is happening – the alignment rings are “taller” than the available space in the manifold guides. A remedy for this is to either not use the rings or to grind them down (recommended). That way they will not be keeping the manifold from mating properly.