You are better off heating the plug instead of the housing. Anytime you heat steel red hot, you bake some of the carbon(which gives steel its strength) out, so it weakens the steel. When you heat the plug, it tries to expand but it is clamped by the housing. Once the plug gets red hot, it yields and relieves the pressure from trying to expand against the cold housing. After the plug cools, it shrinks(contracts) and gets clearance between the plug threads and the housing threads and then it comes right out. The heating also breaks loose any crap(rust, paint, etc.) in the threads that might be holding the plug in. If the plug gets soft or ruined-easy to replace.