I need to replace the piping underneath my sink and the union I bought does not thread up with the old existing part. If I use a new 1 1/2" coupling, will it fit snuggly with the old existing 1 1/2" pipe or have the sizes changed over the years. I will have to cut the pipe and install a coupling and I need to make sure that the coupling is the same size as the existing pipe.

Is it the same schedule, rating as the old pipe?

Old residential PVC will mate up exactly to new pvc pipe.

MARK

Well, I bought a U shaped piece(the little trap part) and it was connected to a threaded union and that would not thread up with the existing male part already under the sink. It is the part that goes from my garbage disposal to the exit pipe. The newer parts' threaded locking piece was bigger than the existing male counterpart and would not thread up so now I have to remove that male adapter and put a new one on to thread up with my new part. Does that make sense and that is why I ask about the pipe matching up since the threaded part does not. I don't want to cut it off if it isn't going to take a new male adapter snuggly.

Can you snap a picture of your sink drain plumbing and post it for us ? Thanks.

Is this the fitting you bought? (see image). Is it the nut that connects to the trap tail going into the wall that's giving you a problem? Back to you, Tom

Yes, that is what I bought and it would not thread up on the 90d elbow coming from the wall, but I just cut it off and installed a new piece(90d ell) and the entire section you have shown here and the garbage disposal into the top part of your pic. It all fit together nicely. Thanks for all the help from everyone.