happened to me about two weeks ago,
tried heating it up with propane and oxy, also tried 2 nuts together and welding a nut to the thread, ( 2 atempts). could'nt get a stud extractor on, none of these worked.
ended up taking the head off for an engineering company to drill out and fit a helicoil.
new head gasget / stud / cap nut.
all back together.
After all that I presume you used Stainless Stud?
