But if it's rounded, it won't come off, right?..That's how I knew it was rounded. So u can use the pipe wrench to remove the old one before u put the new one on. Actually using the pipe wrench to get it off and put it on ... not too tight!...makes me think I won't have to replace it...just don't over torque it and use anti seize if needed.

 

Well just use a pair of Vise-Grip tighten them as tight as possible clamp them and turn the cap off it should only by installed at 18 INCH POUNDS not very tight.

 

It sometimes will take more than the 18 ft lbs to break it loose, because the cap tends to "freeze" from running. As recommended by other(s), buy a new cap, then use the vice-grip, or a pipe wrench if you have a small enough one, to get the old cap off.