If its rounding the corners you have a crummy wrench. Usually a large pipe wrench (and cheater bar if necessary) will do the trick.
You can get them out with a hammer and chisel but this is a last resort.

Thanks for responding. It was a big pipe wrench that I was using like this...
http://en.wikipedia.org/wiki/Pipe_wrench

Then I tried a large crescent wrench. I think what I need is something that doesn't adjust like an open-end wrench. That way I don't have to worry about it loosening on me. Unfortunately I don't have one that large and didn't see one at the hardware store.

I was wondering if heating the pipe around the plug would work? Maybe it would expand and loosen it a bit.

Steve S.

A 3' piece of pipe, large enough to slip over the handle of either wrench, will provide for a lot more torque delivered to the plug than hitting the wrench with a hammer.
Liquid wrench, not WD-40, and heating the fitting with a torch might help some, but leverage will work best.
Don't hit the fitting hard with a hammer, it is cast iron and can shatter.

It can break the cast fitting if you get too crazy. Be very careful with a hammer. Kroil works well to penetrate the threads.

Just took one out of a 100 year-old house, used a 24" pipe wrench with a 3' breaker bar and a sledge hammer. A regular hammer doesn't have enough weight to do the job.
The pipe did turn a 1/8 turn before the plug came out.

Thanks again for the responses. I'll see if I can find that Kroil at the store. I think I'll try heating it with a torch this evening to see if that will expand the surrounding pipe enough to loosen it.

I'm reluctant to use too much force because that square "nut" is really starting to get out of shape. It appears to be an odd sized plug. It's not 2" or 2.5", it appears to be 2.25" and I don't see those anywhere at the store. I can't afford to destroy this one.

Steve S.

Stop!!! Is this a brass plug or a galvanized plug?