A threaded cleanout plug is always preferred but If the threads of the cleanout adapter are too far gone you can use a Lead Fit-All.
You must first have the right size. You will note that the lead has threads in it and you need to thread it in then give it a thump square in the center with a hammer to set it.
You do not just hammer the crap out of is or, you will find yourself reaching into the trap to pull out the pieces and end up buying a new one.
thanx i have a 4 inch trap & i bought a 4 inch fit all and i would think it would drop in there slightly but it doesn't the trap is like 75 years old do u think the new fit alls run slightly bigger or something???
the cast iron part of the fit all is almost the size of the opening ! maybe i should get a 3 3/4 if they sell them
Redwood has it! 3 1/2" is a common clean out plug in old cast iron pipes.
You local home dumpster or likewise store will have that size.
A big hammer will be the correct tool to place it back in as your threads
have been gone on this pipe for a *very* long time.