House trap caps stripped

Hello,
I have a house trap in my basement that leads to the city sewer system. The trap is covered with two caps, but the threads in those caps have been stripped so that they no longer seal.
Sure enough, last year the city sewer backed up due to an obstruction under the street, and some sewage was able to seep into the basement because the house trap caps were loose.
After cleaning up that mess, I would like to prevent it from happening again in the future, so what is the best way to re-fit the stripped caps to the house trap so that they are sealed?
A plumber I spoke with said that it would be impossible to find new matching caps or re-thread the old caps, so he suggested pounding a lead cap into the trap to seal it. Other homeowners I have spoken with reported that they had solved the problem the same way, but it seems like kind of a kludge to me.
Another plumber suggested installing a "Fit-All" plug that is made out of rubber and tightened with a wing-nut, but I can't find any reference to such as part on the web...all the "Fit-All" plugs I saw were made out of lead, so they seem to be designed for pounding in.
Does anyone have any suggestions for the best way to refit these house trap caps?
Thanks,
Russ Mercer
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snipped-for-privacy@byke.com (Merc) writes:

Try looking for a test plug. Rough-in drains have to be filled with water to the top of the vent line to test for leaks. Plumbers use test plugs at the outfall and at each fixture connection to seal up the plumbing for the test. As you describe, they have a rubber gasket and are tightened with a wing nut. Any plumbing supply house will have them.
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Either of the suggested ways should be fine. You can get the 4" rubber compression plug at home-despot, or any plumbing supply store. Or you can make your own out of any 4" soft-rubber cork, pair of big washers, and a bolt. Pounded lead used to be the standard mechanism for cast-iron drain fittings, so there's no reason to reguard it as a kludge. If you don't like either of those solutions, cut out that section of drainpipe, and stick in a new Y..
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Ask your plumber about installing a one way valve to prevent backups in the future. Can a good one with a threaded cover and not one that uses screws to attach the cover (which is worthless when the threads rust).

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Merc writes:

Test plug caps are not made for permanent installation and surely won't pass code, nor would I trust one in the long term.
You might be able to fit a plastic extension using a Fernco type rubber coupling with the hose clamps. This is a permanent code-approved method although the rubber has a lifetime less than cast iron.
If the threads on the pipe (not the plug) are OK (since you considered a new plug that was not to be found), then you could cast a new plug out of Bondo right into it. Bondo shrinks a bit on setting so you could unscrew it and apply some pipe dope or teflon tape. While such an improvisation seems like it could work well, you might not convince the code authorities.
Again if those pipe threads are OK, a good machinist could measure and make a matching plug out of steel or aluminum. Probably cheaper to cut out and replace the old sections with Ferco's, though.
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