Model A & B exhaust manifolds are well known for warpage and sealing problems and this page explains some of the issues and causes.
The basic manifold design has a couple of shortcomings which lead to problems. First, there are only four attaching studs and there are no attachments at the front or rear extremes of the manifold.
This bolt pattern allows an uneven clamp load which can allow leakage at the unclamped ends, as well as differential movement to the block with manifold expansion and contraction during temperature changes.
Second, the exhaust outlet is located at the far rear rather than at or near the center of the manifold.
The position of the outlet at the rear means that all four cylinders' exhaust heat is transfered and saturated at the rear port area. In contrast, the area at the number one port 'benefits' from cooling air from the radiator fan and the heat from only the number one cylinder.
This uneven distribution of heat along the manifold, combined with a less than ideal attachment pattern, allows the manifold to expand and contract unevenly during heating and cooling cycles.
Additionally, the weight of the muffler is suspended from the rear of the manifold, and adds an ever-present downward force on the hottest portion of the manifold.
Now just for fun throw in the fact that a great many Model A's also do not have proper ignition timing, and owners like to idle them at very low rpm's and tend to drive with the spark too far retarded.
This late timing condition causes fuel to continue burning and releasing heat after passing into the exhaust manifold. This adds tremendous heat energy into the casting and actually can make the casting glow red hot as many people have experienced.
Shown above is the droop or sag at the number 4 port on a Model A manifold. The first three ports remain in line, but the number 4 port has dropped significantly.
This type of warpage is the primary failure. Manifolds do not seem to change shape or warp in the machined gasket surface plane. Re-surfacing the gasket surface is really of no value in addressing the typical warped manifold.
The up/down alignment of the four ports is really the concern. Typically the first three ports remain in a straight line, and the fourth port drops 1/8 inch or more.
The droop or sag at the number 4 port on a Model B manifold. The first three ports remain in line, but the number 4 port has dropped some.
If the port drops 1/8 inch then it is very difficult to align and seal the manifold and gasket to the engine block. The likelihood of an exhaust leak is very high.
Sag of the rear port by more than 1/8 inch is a good signal to start looking for a replacement manifold, or be prepared to deal with exhaust leaks at that area.