I had a problem with my sewer being clogged. I dug-up the clean-out and removed the cover (it was brass) from the cast iron clean out. I tried to replace the cover but the threads were gone nothing left. I would like to re-tap them so I can put a pvc male adapter in to replace the cover. How do I do this?

Why go to the bother? Simply purchase a new cover in brass or plastic. Good luck, Tom

It will not thread on. The threads are rusted out. I need to re-tap, or something. That is why I am submitting my problem. Is there a tap for this? Is there an insert?

This is what I think I would do. This is a makeshift repair and may work or may not.

You will never find a tap big enough for a clean out and if you did, it would cost you a fortune. I have never heard of an insert for a clean out.

1. Put a small wire brush in a drill and clean the inside of the clean out well.
2. Purchase a PVC clean out plug, some J-B Kwikweld, some wheel bearing grease.
3. Coat the threads of the PVC plug generously with the wheel bearing grease. Wipe off the excess.
4. Apply a generous coating of J-B Kwikweld to the inside of the clean out and to the threads of the plug.
5. Install the plug in the clean out.
6. Wait 4 hours for the J-B Kwikweld to cure.
7. Try to unscrew the plug. If you can unscrew the plug, you are finished. If you can't unscrew the plug, try heating it around the edges with a torch and then try unscrewing it while it is still warm. If that doesn't work, cut out the center of the plug, close to the edge, with a jig saw or hacksaw blade. Then remove the edge that's left and install a new plug.

This is a unconventional repair, if it doesn't work all you have lost is about $15 and an hour's work.

Go into a plumbing shop and ask for a 4" Fitz-All cleanout plug. This is a metal plug with tapered threads that's made for problems such as yours. If they don't have it try a 3" snap in replacement cleanout cover,(see image)
They can be seen at, PVC Snap in Clean Out (Hangzhou Yimen Hardware Manufacturing Factory,China (Mainland)) (http://yimenhardware.en.alibaba.com/product/50088850/50414001/Plastic_Pipe_Fittings/PVC_Snap_in_Clean_Out.html)
Good luck, Tom

Another makeshift approach might be to purchase a 3inch FPT (Female NPT) x 3 spigot shown below. J-B weld the female end, whose outside diameter should be 3.5" or greater into the clean out opening. Then cut flush with clean out and install a 3 inch plug.