I found a good discussion on this topic at DIY (http://www.diynet.com/DIY/post/1,2021,3_175033,FF.html. Hope this helps.
After this, I used the cut-off wheel to slice away portions of the cap until I could get near the inner ring. I then cut slots right up to the edge of the cast iron pipe.
From there I used a hacksaw to cut vertically from the inside out to the edge of the threads. You can feel and hear when you start grazing the cast iron, so cutting by hand this way is safe. Once I'd cut through the plug in about four places, I tapped the chunks out with a cold chisel.
I don't have a replacement plug yet for the odd 3-3/8" cleanout, but I'll go hunting today. I was amazed at the amount of crud in there -- a big plug of junk and sediment lining the walls of the cleanout.
from a woman: this plug/cleanout/cap in my house probably has not been removed since the house was built in 1938. i couldn't get it unscrewed. it had been painted over and blah, blah, blah . . .
because of your post i got a heavy duty short sledge type hammer. i gave it some hard bangs. not only on the side of the pipe on the end where the cap is but also on the head of the cap. i then tried to unscrew. still too hard. i could not. so i got my feet centered firmly on the ladder and had the pipe wrench vertical like a gym bar with both hands firmly gripping. i slightly bent my knees and lost contact with my feet and the ladder. all my 130 lbs. was hanging from the pipe wrench. the cap unscrewed!
OK, now you have a hole. Take a sawzaw and cut a 1/4 of the pie out. Careful not to cut into the threads to deep on the other side. Remove the pie with channel locks and a chisel. Using the chisel and a hammer, crush the remainder of the cap inward to release the threads. At this point you should almost be able to spin it out by hand.
I am a plumber and do this weekly, it takes me about 20min. Replacements can be found at a hareware store or your local plumbing supply house. Expansion plugs aren't bad either.
Now I hope to find a replacement.
Using a drill bit to starter a hole, then my jigsaw with a metal blade to cut the cap into 3 or 4 sections so the threads cam collapse inwards.. Hopefully have it out today or 2moro!