Whether you are installing a new bathtub or repairing an old one, bathtub drains need attention from time to time. Overflows rust out. The stopper mechanism can break from age. Old gaskets and seals may begin to leak. Whatever the reason, replacing a bathtub drain is job you can do yourself if you have a small amount of plumbing skill.
Begin the job by locating the access panel to the bathtub plumbing.
If you have a free standing tub, this is not an issue. However, most homes have bathtubs that are built into the house. Usually, there is an access panel on the wall that the tub rests against where the plumbing is located. It frequently is at the back of a closet in the adjoining bedroom. If there is no access panel, you will have create one.
Use a small saw like a keyhole saw to open up the wall so that you can reach the bathtub plumbing fixtures.
This will work best if you can locate the studs in this wall. Cut the opening near the floor directly behind the tub fixtures that reaches from one stud to the other. You will need an opening about 20 inches long. Try to cut the drywall all in one piece. That way you can use it to close the panel when you are finished installing the drain. Set the piece of drywall aside and make sure that you have a clear access to the plumbing and the underside of the bathtub. If you missed, you will get to learn how to repair drywall after you locate the plumbing.
Looking from the back of the tub, you should be able to easily see the bathtub drain.
It will look like about a two-inch tube running from the overflow down and under the tub. Go into the bathroom and unscrew the cover for the overflow tube. If it has a mechanical stopper, you should be able to pull what is left of it through the opening in the tub. The lower part of the drain should be held in place with a screw that you can remove. Lift the screen away from the drain. Clean off any old gaskets or sealer at both the overflow and the drain opening.
Return to the access panel and pull the drain loose from the tub.
Do not get too carried away with this. The drain is still attached to your house plumbing. You just want to make sure that the drain is completely loose from the bathtub. The drain should have a tube running from it to connect into the house plumbing. Look for a large metal or plastic nut that holds it together at the point where the bathtub drain meets the other plumbing. This may be under the tub or in the basement or crawl space of the house.
Unscrew the nut to release the tub drain from the plumbing.
You may need a pipe wrench to do this. However, these can often be released by hand. Be gentle with the pipes in the house. You do not want to expand your plumbing job by damaging other pipes. The drain should be able to be lifted out of the plumbing or the plumbing pipes pushed away from the drain. This should fully release the bathtub drain to be removed.
Compare the old drain to the new one.
You want to make sure that it is exactly the same size and design. Ease the new bathtub drain into place. Slide it inside the house plumbing, but do not tighten the nut. Align the overflow and drain tubes up with the tub openings. When they are in position, return to the bathroom.
Slide the stopper mechanism into the tube and fasten the overflow cover and gasket into place.
Do not fully tighten these screws yet. Put the drain cover in place with the appropriate gasket and screw. Tighten this screw until it is just snug. Finish tightening the overflow cover.
Return to the access panel and finish tightening the nut on the plumbing pipes.
Make sure that the covers are secure and tight on the tub. The bathtub drain should not be able to be moved if everything is secure. Run about an inch of water into the tub and check for leaks. If you have any, try tightening the drain a little. This should take care of it. Test your stopper to make sure that it works. These rarely have an issue because they are made to fit at the factory. Repair the access panel by framing it in or tacking it into place.