Big Tracks---I'm with you---That heating something to make the hole get bigger is hard to get your head around. If you think of it in terms of heating the metal around the hole, it seems like the metal would expand and make the hole grow smaller.---But----It doesn't work that way. Exactly the opposite in fact. Think of the hole as having a circumferance of a given value. When you heat the metal, the metal expands, and the circumferance has to get longer because the metal is expanding. When the circumferance gets longer, then the diameter has to get longer as well, and it does.