I can not get a new 3 inch cap to screw into my cast iron clean out opening. I cleaned tit with a dremmel wire brush and WD40. Still a new plug will not start, the old plug will start and go in a ways. Is there a way of repairing these threads or are thiere different thread tyoes? The pipe was instaled I n 1972.

Go into a plumbing shop and ask for a 3" 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))
Good luck, Tom