The seal is "ceramic". It seals exhaust gas fine, but adds little structure. It is slightly porous, but soot soon clogs the pores. It is quite friable, so when you start twisting the parts it breaks up and falls out of the joints, like sand. That is why it is easy to remove and keeps the two metal surfaces from rusting together. "Furnace cement" is similar material. The highest temperature epoxy I have seen is 750 F, which wouldn't last in the exhaust under worst conditions. Normal epoxy ~250 F, silicone rubber ~500F. Any cement or sealant >750 F is usually ceramic.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans