I have successfully isolated a clog in my main sewer line (the right side of the whole house drain) I opened, cleared, and then replaced the two plugs at the top of the 'U' joint. I unscrewed but unalbe to remove the service clean out to the left of the 'U' joint between the 'U' joint and wall. I noted that it was fitted with a "warco 4" xd neoprene dual tite" gasket. It doesn't see to have the normal threading required to screw it back in place. My guess was to use a rubber mallet to wedge it back in but I fear breaking something so I come to you. How do I replace this plug and reclose my sewer system?