First off a tap that big will cost quite a few $$. Next you will have to use a finishing tap, not a tapered starter. If you can't or won't get one, try to get an old scrap injector, slice or grind 3 or 4 grooves into it like a tap has, and use that. You don't really need a tap that is hardened to cut new metal, just something to clean up your threads. Just start it in straight and turn it in back and forth. Also cover it in clean grease so it will catch the filings