If you have an endless zip or a piece that you've cut of because it was too long, you can put a new slider on and make a closed end (non separating) zip for a pocket or fly.
Please order a bottomstop for the end, to be inserted through both zip tapes.

When replacing a slider on a zipper, you need a topstop, or actually two topstops. Use pliers to pinch the topstop on the band of the zipper where the teeth end.You new slider can no longer come of.