Old 2-flat main stack clean-out problems

[chadzeilenga]

Hello,
I recently purchased an old (100yr) 2-flat apartment building that I am having some minor plumbing problems with. I recently had to have a plumber come and rod out the sewer line to the city sewer system because the building was vacant and tree roots had clogged the system up for the new tenants soon after they moved in.
We got all of the roots out and the water flows great now, but when we removed the PVC cap from the clean-out on the cast-iron main stack I noticed that it wasn't threaded on very well, just sealed up with putty. After we were done I tried to thread the plug back on it would not thread. I looked closer and saw that much of the threads on the cast-iron female section were worn away probably from previous sewer rodding and the previous owner just used sealant to hold the cap in place.
Is there a way to repair this without cutting apart the main stack and breaking up the concrete floor to replace the section of pipe with the clean-out on it? I am thinking that there might be a plug similar to that on a row boat which you turn a center screw and it expands the rubber outer part to seal the hole? I don't think that the threads are dirty, they are just worn away.
Any suggestions?

Thanks,
Chad

 

[Gencon]

An item called a test plug will work here, although I'm not sure if they're allowed for permanent installation.They are normally used to temporarily cap off a drainage/vent system while work is being done. They come in various sizes and are basically a large rubber ring with a large washer on each side.There is a bolt running up the middle and a wingnut on the outside. You insert the device in the pipe and tighten the wingnut. This compresses the rubber ring and forces it outwards to seal against the pipe.

Or you can try a new PVC cap with fresher threads.

 

[Sponsor]

 

[hj]

plug

The threads disintegrated because there used to be a brass plug in the opening, and that happens frequently. The problem is that a test plug is designed to fit inside the pipe so it will normally be about 3/8" too small and most will not expand that much. But the plug may be 3 1/2" pipe size in which case a 4" test plug will work.

 

[chadzeilenga]

No PVC allowed...

What is currently in the pipe now is a 3.5" PVC plug that I have hammered in place. It has no threads left. The previous owner installed it. I bought a new one and tried to thread it in there and it would not go, kept trying to cross thread itself and fall out. I think that the threads on the PVC cap are deeper/taller than those on the brass plugs???
I tried the 3.5" test plug, but by the time I had tightened the wingnut enough to get a seal, it broke. It was made out of all plastic and I think it was intended for use inside of a smooth PVC pipe.
I'll try the 4" test plug, didn't even think of that. I'm going to put some sealant around the outside of the plug so that it goes into the threads because it didn't feel like the rubber on the outside was pliable enough to get itself inside of the threads and create a good seal.
Let me know if anyone can think of anything else...

Thanks,
Chad