Being a model shop guy u are going to have a tough time getting those threads good unless u pull the head. All a charger is is a dull tap. U don't want to use a new tap because u will take more material out of hole. The best way would be to pull head and work at it from back side.if the threads r crossed already chances are that you are going to loose the first couple. Then there is the compression issue. A lot of it. Now you have a thread you tried fixing from top that has slop in it. Well someday that plug is going to blow out when your out. Take the head to a shop and have them fix it correctly. They may put a heli-coil in . Good luck and use never seize when assy. John