Grind the first 2 or 3 threads off the tap. Re-insert it into the hole you've already tapped, and work it in about 1/4 turn at a time with lots of lube. I've got a a set of modified taps I use for threading the ends of oil galleries on smallblock V-8's. The hole needs to be opened up to the size that will allow the plug to engage several threads. Use a sealer, or LocTite, and don't over-tighten, as you can crack the cast iron with a too-tight tapered plug.
Jerry
