[Wavery]

Using brute force on that plug can have drastic consequences.

I have found "PB Blaster" to be a very effective penetrating oil. Tapping is certainly a good idea. Instead of using a hand wrench, I have used an impact wrench with great success over the years. It exserts more force with less likelihood of damage. It also helps break up the small bits of corrosion and crystalized hard water deposits that are wedged between the threads.