The purpose of a lock nut is to hold the sink drain assembly secure in the sink above. If you're replacing a sink drain and find yourself facing a stuck lock nut, there are a number of different techniques you can use to help remove it. Don't get frustrated if one method doesn't work, just move on to the next. Eventually you'll be able to remove that stuck lock nut from the sink drain.
Things You'll Need
Spray bathroom cleaner on the stuck nut and wipe up with a towel or rag. This will eliminate any mold or mildew growing on the surface and allow your tools a better grip.
Spray the nut with penetrating lubricant. Allow this to soak in according to the manufacturer's instructions.
Tap the side of the lock nut with a hammer. Repeat on all sides of the lock nut. Loosen the nut with a wrench.
Clamp a pair of locking pliers around the nut and lock into place. Tap on the end of the pliers with a hammer to drive the nut counterclockwise for removal.
Heat the nut with a plumber's torch. The heat will expand the screw, making it easier to remove. Loosen the nut with a wrench.
Slip a nut splitter over the lock nut. If the p-trap on the sink is not already removed, this will need to be done first. To remove a trap, place a bucket on the floor beneath the trap. Loosen the couplings on either end of the trap with a wrench. Pull the trap out of the sink plumbing and empty it into the bucket. Slide the nut splitter over the end of the tailpiece and up until it is over the lock nut. Tighten the nut splitter with a wrench until the lock nut splits and falls off.