New sink drain is 1 1/4 ptrap is 1 1/2 both have to stay

Not sure how a bathroom sink trap ever got to be 1 1/2 to start with and not sure of your question. Could you further explain?

Hi Lordacton...

All lavatory pop-up drain assemblies are 1 1/4". Most drain pipes to bathroom sinks are 1.5".

Your old trap should have some kind of threaded adapter on it... what size is that? If it is 1.5" then you can purchase an 1.5"x1.25 nut and washer and then purchase an 1.25" tubular extension pipe.

Otherwise, I will need to know what kind of pipe you have to instruct further... let us know more.. OK?

Mark

new sink drain is 1 1/4 ptrap is 1 1/2 both have to stay

If the trap's metal they make a 1 1/4" trap.
If you have a 1 1/2" PVC trap there should have been a 1 1/2 X 1 1/4 nylon gasket with the new trap. If not pick one up at your local plumbing shop. Good luck, Tom