Not just any tap, but a 3/8″ NPT pipe tap. In my son’s 400 Pontiac block, threading for oil gallery plugs.
The trick to remove them, is to weld (preferably TIG) the tap itself to the inside of a large nut. Keep a LOT of heat on it. The heat will try to expand the tap, but because of the cold iron block surrounding it, it can only expand lengthwise. Then let it cool – but it cools and contracts both lengthwise AND widthwise!
Ta-da! Crisis averted!
