I admit that verifying the fill plug is a good idea.
I've never had a problem getting a fill plug out. There are three reasons for this:
1. The Hulk has it right: Get angry, get strong. The genuinely-difficult part of this--and what The Hulk does not get right--is getting angry enough to be effective, while not losing your mind.
2. When dealing with female square wrenching surfaces--as commonly used on iron pipe-plugs--DO NOT EVER try to use a 1/4, 3/8, or 1/2 inch drive socket extension or ratchet/breaker bar square drive
poked directly into the female square hole in the plug. Those tools are drilled for a spring-loaded ball or pin to retain a socket. They will break at the drilling. Use a dedicated plug removal tool of the correct size for the female square hole in the plug. The dedicated, proper tool will not be drilled for a socket retaining ball.
For example: (I have not used this set, and there's many other brands that have essentially the same tools. I bought mine one-at-a-time to fit the plugs I was dealing with, not as a "set".)
www.amazon.com/Grey-Pneumatic-1308P-8-Piece-Socket/dp/B00KBYJVUA/ref=sr_1_9?crid=3E0VL9IJC07AD&keywords=pipe+plug+tool&qid=1640210281&sprefix=pipe%2520plug%2520tool%2Caps%2C234&sr=8-9
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3.
God bless Oxy-acetylene. Heat the PLUG cherry-red, let it cool, screws right out.
Propane or MAP-Plus is not hot enough unless you have a proper torch and a bottle of oxygen to go with it. (Oxy-Propane, or Oxy-MAP-Plus) Heating the PLUG does two things--it becomes pliable/malleable/deformable, and it expands from heat. The plug expands, but cannot expand in diameter because it's jammed into the "cold" casting. Therefore, since it's pliable, it expands lengthwise. When it cools, it contracts both lengthwise
and in diameter--making the diameter smaller than when you started. Reduced diameter breaks any rust bond, and increases the gap between male and female threads.