On the main water line to my house, there is a black gasket that seems to be leaking. What is that black nut on top? Have you seen this kind of thing before? I want to know if I can just tighten it with a wrench or if I need to turn of water line and replace it.
The picture turns out to be a picture of a test cock on a backflow preventer. The black plug is just cosmetic. One the side there is a way to use a flathead screwdriver and turn off the test cock (in my case, it had been slightly open).