[Stefan]

Worth trying a new exhaust ring - it might have enough meat to bite on the tired old thread - and you could clean out the cast iron threads carefully with a file.
I made up my own rings from brass stock on the lathe once. But don't make the same mistake as I did; after a fast run the exhaust blew again and both rings had drooped and distorted, in a surreal, Salvador Dali-esque style. I think the correct material should be bronze or bellmetal, something with a higher melting point. Engineers please advise!