Picture: [postimg.cc]

It is a 1 1/2 inch metal washer on top of a rubber washer, pressed against the 2 1/4 inch vent pipe by a nut threaded on a bolt through the washer. The bolt does not go through the pipe to the other side, so it must be anchored on the inside of the pipe. It is about 5 feet up from the basement floor.

I have loosened off the nut enough to raise the rubber washer and see that it covers a hole that is slightly smaller than the washer itself, but there is so much gunk in the pipe that I cannot see how the bolt is held in place.

If this is an access port for snaking, I would like to open it up, but I do not want to drop any parts that hold the bolt in place down the pipe (if it involves springs or other small metal parts, it is likely they are corroded.) Do you have any advice about how to go about this?

It is an expansion plug. Loosen the nut and the rubber seal should contract enough so you can pull it out. DO NOT remove the nut.

Hi hj, thanks for replying. In the picture I posted, it does look just like an expansion plug, but after I loosened it off some more, I could swing the bolt to one side and dig the gunk out. What I found was this: [postimg.cc] - I guess originally there was a circlip on the pin to stop it falling out while you install it. Thanks for mentioning expansion plugs, as that looks like the best thing to replace it with.

It is a "tank plug', not an expansion plug. The "crosspin" went through the hole and then, when the nut was tightened it compressed the washer against the surface to seal it.