How can a cast iron cleanout with rusted stripped threads be capped?

Answer 1

well you can either have a plumber come and redo fitting or its possible you might be able to get a rubber style cap that will fit snug around it and install it with a band clamp-of course the second being the least expensive way-other would cost hundreds of dollars

Answer 2

There is no pressure on a cast iron sewer drain. Spread silicon adhesive where the threads used to be and press it in place. It will be good until the next time you need to use it (15 years?) then just pry it out and re silicone it.

Answer 3

Get a rubber test plug that fits the opening.