I have a cleanout plug behind a finished wall in my bathroom. There is a Steel coverplate covering the cleanout. One night I noticed while the washer was draining I heard a jingling sound. It turns out that the screw holding the cover on had rusted out. Basically the setup is the screw goes through a hole drilled in the cleanout and goes into the pipe. I am not sure what the screw actually attaches to, but any new screws I have tried to put in there are not holding and therfore are not keeping the cover on. Temporarily I have siliconed the hole so it doesn't leak, but that doesn't allow me to put the cover plate back on. Is there something that I am missing? How do I close it up, put this cover on and make so it doesn't leak? If this is too vague and you need a better explanation please let me know and I will do my best as right now I am scratching my head about this!
Quick note... this is PVC drain piping!