So I have this old beast with 90 460 in it.. Kinda a bita truck, bita this bita that. Engine is bone stock to the best of my knowledge. EFI . My problem is broken exhaust manifold bolts. 3 to be exact. Got one out ok. Well a royal pain but enough threads left to be functional. #2 not so good. Drilled it out to helicoil the bolt, but the tap doesn't seem to want to start. Doing this by hand, stuffed under the fender. The tap will just catch, then chip off the piece and keep spinning. Its actually just hogging the hole out. Are these heads high nickel content or something weird I am not aware of. I've hand tapped more than a few holes and this just doesn't feel right. Put a lot of heat there to attempt to take the temper out of the hardened bolt. That shouldn't harden the cast, right? I'm really frustrated. Don't want to mess up the hole and REALLY don't want to pull the head. Any info is appreciated . Thanks.