Titter ye not!
So seven years ago I put a 3KW immersion in my 650L tank and on the end of the thread inside the tank put one of those 2.25" BSP smooth solder flange nuts on the inside of the tank wall.
But I cannot undo it as it has probably corroded on the thread and I don't want to use loosening oil as it's on the inside of the tank.
Any ideas?
It is smooth and round with no "nut" faces at all. I did wonder about getting a large jawed set of mole grips or using a pipe wrench - but I can only get one hand inside the through the tank opening.