I have a cast iron sewer pipe riser that is 4". the top of it is a hub, inside the hub is a rubberised gasket (I think some calls it a donut). Inserted into this donut is a metal cylinder adapter with threads on the inside where the brass cleanout plug screws on to. The plug and the threads are all corroded and gone, I cut the plug out and now I am trying to pull off this metal cylinder and the donut but the donut won't come out. I cursed and cursed and pryed and pulled and it just won't let go.