I also have used alot of PB Blaster and a pipe wrench with BFH , but i had an engine that someone threaded a short piece of pipe into the exhaust outlet and the pipe would collapse with a pipe wrench.
I took a sawzall and cut a slot thru the pipe and collapsed the pipe and it came right out !!
I knicked the threads but not enough to where the the elbow wouldn't tighten up. Use a new lock ring on the elbow.
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