I demolished my old kitchen today. Since my new one is in a new location, I tore out the old cast iron oakum stuff , cut the last of the 2" close to the 4" main I am reusing, and installed a gem cap. Is that kosher?
BTW, that old 2" C.I line looked like a 3/4" pipe inside. No wonder I had issues for years.
If I read your description properly, you used what we call a "dollar plug". Our "gem caps" are a rubber cap which goes over the pipe and secures with a hose clamp. If you DID use a gem cap, then I would not trust any rubber cap inside a wall. They do deteriorate, which is why we have to scrap our old ones periodically.
It is indeed a rubber Gem cap. But it is in the crawl space, on a 2" branch, about 5' from the main 4" run the length of the house.