The sewer line has a wye with a 4" clean-out plug. The plug came off and won't go back on. I purchased the Cherne 270188 clean-out 4" mechanic gripper plug (black). It was too big. So then I bought the Oatey 33403 plug (red) and that was just too big. I started to file the sides, but my file is very fine, so I stopped and will get a better file tomorrow. On a side note, the clean-out plug is located in a really tight space, so I don't have room to apply more force. A couple of inches from it the vent pipe goes up, which you can't see in the pictures. Tight by reg posted Oct 18, 2019 at 12:09 AM If anyone can suggest a simpler solution it would be greatly appreciated.