The material used to foundry cast the exhaust manifold was good for the day, but did not have the metalurgy used in exhaust manifolds of late. The Dowagiac foundry's castings streessed from prolounged contact to the hot exhaust gasses (both in terms of temperature and chemical content of the exhuast gas) and there were points in some castings that were weaker than others. The cracking/breaking problem was noted in 1951 and 1952 Kaisers. No it is not a chronic thing, but rather, something to watch for as time goes on. If you have one of these cars and can get a solid spare (you can take it off a donor engine or parts car) you might want to have it around just in case.
The British were good ones for high-temp weld repairs to manifolds, but if it does work initially, the weld is yet another weak point in the casting.
