by hennesse » Sat Oct 06, 2018 8:45 am
I agree with Mutt. The petcock is some kind of pot metal (zinc, aluminum, and whatever), so it's a little softer than steel, but not that much. The best thing is that it doesn't rust, so it's not rusted to the tank threads. It's just been in there for 60 years, and it's reluctant to leave.
Your only real choices are heat, penetrating oil, and the method you suggested. Your method sometimes works easily, but sometimes the male threads are firmly stuck to the female threads, and the "prying out" ends up causing damage to the female threads - which is what you're trying to avoid.
Penetrating oil might help, but I suspect it is not worth the effort. You'd have to drain the gas and replace with pen oil. Tap, tap, tap the petcock many, many times in hopes that a little will find its way between the threads. But since gasoline doesn't, why would oil? Pipe threads are designed to seal better than straight threads - that's why they use them on pipes! Remove the tank, flip it over, and repeat from the outside. After tapping hundreds of times, you most likely will have accomplished little or nothing.
Penetrating oil and heat combined are often the best method for parts that are really stuck together. Heat the joint where the parts meet, and spray with penetrating oil. Continue spraying as the parts cool. As the internal and external parts expand and cool at slightly different rates, a tiny bit of space may open up momentarily between them - enough to allow a little pen oil to seep in. I remember trying to get the end caps off some rusty Knucklehead handlebars without hurting the bars or the caps. Had to repeat the heat/spray process about 30 times, but finally they came apart without any damage to either.
I've taken the petcocks out of maybe a half-dozen Harley gas tanks in my life - just brute force effort with a big wrench. They always came out without damage to the tank. Sometimes I damaged the wrench flats on the petcocks, sometimes not. But since you don't care about the petcock, just grab a wrench!