On Thu, 25 Aug 2005 20:24:29 -0700, DaveC wrote:

I've got an old 20 gal (?) compressor tank in which I want to install a
drain. I'm guessing it to be 1/4-inch (6.3 mm?) thick steel. Tank is rated to
125 psi.

Can I drill and tap this for 1/4 or 3/8-inch pipe? Or should i get a "plug"
welded to it that I can drill & tap?

It won't be thick enough to tap. Have a plumbing fitting welded or
brazed in. I'd braze it. That's what I did on my tank.
Plumbing fittings are often cast iron which can be welded to steel --
but brazing is a lot easier, would have ample strength and is less
likely to have pinhole leaks or cracks.

Are there rules to follow for such stuff? Is there a better n.g. to ask this
question in?

There are rules for damned near everything, but if it's for your own
use I wouldn't worry about it. As Jerry noted, there isn't much
hydrostatic force on a small fitting like that at 125 PSI.