Tried one when I needed to remove a seized injector on a Peugeot HDI engine.
Absolute waste of time, all it did was to soften the injector so it sheared off when we put a bit of pressure on it. Ended up taking the head off, setting it up on the vertical mill and drilling out what remained of the injector.
Might work where you need to remove a nut from a bolt where the heat will expand the nut away from the thread but not on a bolt which is tapped into a casting.
Smoke spanner for me every time!
Arwyn
