The metal piece you see in the sink’s drain opening is the drain assembly. Some drain assemblies have grids inside of them, while others may have a pop-up stopper that raises and lowers in the drain assembly to either keep water in the sink or to let it out. A drain assembly does not last forever, and over time it can become pitted or otherwise wear poorly. The drain assembly may also become the source of leaks as the seal between the assembly and the sink itself fails, allowing water to pass through uninhibited.
Loosening a sink’s drain presents a challenge, especially if you have not worked with sink drain’s before. The person who installed the sink’s drain may have used either plumber’s putty or silicone caulk to attach the bottom of the drain to the sink and to create a watertight seal. You must exert enough force on the drain assembly without scratching the sink to break the seal. If your sink’s drain assembly has a grid in it, you may form a makeshift tool to twist the drain assembly out of the sink. First loosen the first pipe connection under the sink, and then insert the handles on a pair of pliers into the sink drain assembly. Place a long and thick screwdriver between the pliers’ handles and use the screwdriver to turn the pliers and the sink drain assembly counterclockwise. With drains that do not have a grid, pull out the stopper and insert a drain key. The drain key has a dial to expand it against the sides of the drain assembly. Use a pair of pliers to clamp onto the top of the drain key and turn it counterclockwise.
A replacement sink drain assembly needs to fit properly in your sink’s drain cutout, otherwise the drain assembly will cause continuous leaking. The best way to find a replacement drain assembly is to take the old one with you to a plumbing supply or home improvement store so you can match the size of the old assembly. The replacement drain assembly does not have to match the old drain assembly in any way other than the size of the assembly.
Before you install the new drain assembly, apply a seal around the drain opening in the sink. You must first remove the old plumber’s putty with a putty knife. If the previous installer used silicone caulk, apply a caulk softener before scraping the old caulk off the sink. Either roll a rope of plumber’s putty and wrap it around the sink’s drain opening, or squirt silicone caulk around the drain opening. Always place all of the seals that came with the assembly over the underside of the assembly before tightening the retaining nut with a wrench.