Always place unit in a good vise and tighten well....On really stuck caps, I use a Makita grinder with a thin cut off wheel. Move it around the area where the cap meets the body. By cutting in as far as the threads, you will heat up the aluminum. In most cases a little Kroils will do the rest? If not, you might have to resort to using a cold chisel around the edges with a bfh. Paul www.severngaspumps.com