Sinks - that is kitchen sinks- use 1-1/2 in. p-traps. In replacing the drain system, there are a couple of things to be aware of.

1. If the installation is an old one, it is likely that the waste arm (the part that goes in the wall) is leaded into place. Today, there is a p-trap called a repair trap that has a slip joint connection. This allows you to take a tubing cutter or a hacksaw to the waste arm and cut off the bend and slip the shortened repair trap arm in its place.

2. The other problem you may encounter in replacing the j-bend is that older ones are usually a bit longer than the ones manufactured today. You may need to replace the tailpiece (the piece coming down from the basket strainer assembly under the sink) or add a slip joint extension to the existing tail piece.

Basins - that is, bathroom sinks - use 1-1/4 in. P-traps. All of the above applies to them as well.