Hey guys I have a 4.0 91 jeep Cherokee when I removed the intake and exhaust one stud broke in the head so. I drilled a whole and put a stud extractor but the extractor snapped so I'm screwd because I can't drill harden steel, but I talked to a machine shop and the guy told be to put muariatic acid ontop of the stud and the acid will eat the extractor but not effect the cast iron is this true????