Myself writes: “When penetrating oil and breaker bars fail, brute force and carbide steel come to the rescue. There’s not much more to say, except that when you’re done you might need a thread repair file.”
Ok, maybe there’s a little more to say. These work by forcing a wedge into the side of the nut, splitting it in half. Yes, it’s desctructive, but there does come a point where off is more important than off and intact. Nut splitters are for those times.