Question About Clean Out Caps...

Hi guys, I'm new to this board & hope to learn some tips from reading the posts. I'm a real estate (small time) investor & contractor in Chicago. I'm not a plumber but I know how to do most basic plumbing stuff & find I'm doing more all the time. If I can't handle it I call my pro. Sometimes I run into things I don't understand & hoping I can get some help here

OK, here's my problem...

I'm cleaning out the pipes under a basement utility sink. I went to open the clean out in the stack & found the cap just nudged in tight enough to keep any water from leaking out. In other words, the cap has threads but the stack hole does not. There's no way to screw it in tight. I guess the last person working on it just smacked it with a hammer until it was tight enough not to leak. Obviously, I don't want to leave it like that

I've never seen something like this. I figured all the female openings in a clean out would have threads. This one is totally smooth. I'm perplexed

Is there a special type cap to use? By the way, it's all steel (or cast iron)

Thanks in advance

They make a twist-in expanding cap that will suffice in this situation. Old threads have a tendency to disappear/disintegrate over time.

Thanks for the reply

Do they sell these caps at Home Depot type stores or do I need to go to a plumbing supply place?

Most hardware stores, Home Depot, etc... will have them.

Measure the inside of the opening. Yours may be 4". Most of those plugs are meant to fit inside a pipe, not a fitting, so you may not find one that will expand enough to seal your opening.