Got your eye on a deeper sink? Measure the tailpiece first!
Measure the tailpiece
You can choose a new sink with a deeper basin than the existing sink has, but if it hangs down too low, it won't drain properly and you'll have to lower the sanitary tee connection in the drain line inside the wall. You'll definitely want to avoid this task if the connection is metal and ends up being behind base cabinets. Plastic pipe is easier—if you can get to it easily. The actual tee connection may be several studs over from where the waste arm enters the wall.
Measure the sink tailpiece between the basket strainer and the tee. That measurement is the extra depth that can be added to the sink bowl without lowering the drainpipe going into the wall. Also be aware that a new disposer may have a lower drainpipe than your existing one—but it can't be lower than the tee. If the disposer drainpipe will be too low, consider a sink with different depth bowls. You'll have a deep bowl for dishes and a shallow one for the disposer.
Make sure the new sink will cover the old hole
Check the cutout size
A sink that's too small for the countertop opening will leave ugly gaps along the sides (or even fall right through the hole!). Before removing the existing sink, measure the opening from underneath. Measure all four sides because the cutout may not be square. Pay special attention to the corners. Contractors often cut them at 90-degree angles (instead of rounding them off) because it's faster.
Take the measurements with you when buying the new sink and make sure it'll cover the opening, including any square corners. If you can't find a sink that'll fit, buy a larger one and enlarge the opening.
Swollen countertops spell trouble
Check for water stains
Leaks around a sink rim can soak the particleboard under a plastic laminate countertop. A little water damage is normal and won't interfere with your new sink. But severe swelling will prevent the new sink from sitting flat on the countertop. And crumbling particleboard won't provide a solid base for the clips that fasten the sink to the countertop.
Look at the countertop surface around the sink. Check for bulges or areas where the laminate has loosened from the particleboard. Then look at the countertop from under the sink for areas that are too spongy to support sink clips or support the sink itself. If you find any of these problems, replace the countertop.
Seal with caulk, not putty
Use the right sealant
Plumber's putty has long been the standard sealant for sink baskets and sometimes even sink rims. The problem with putty is that it eventually dries out, cracks and causes leaks. Worse, it can damage some plastics, including some of the plastics used to make sinks. Avoid drips and disasters by using a silicone caulk instead.
Use a kitchen-and-bath 100 percent silicone that requires solvent cleanup—sold at home centers. Apply a bead around the sink opening when you set in the sink and around the drain opening when you set the disposer drain and basket strainer. Wipe away excess caulk.
Remove ALL the old caulk
Use caulk remover
The caulk around your new sink is all that'll stand between your countertop and water damage. For a lasting, watertight bond with the countertop, you have to completely remove the old caulk.
Remove the old sink, then scrape off the caulk (or plumber's putty) with a putty knife. Apply a caulk remover (sold at home centers) to stubborn caulk. Let the caulk remover sit for a couple of hours, then scrape off the softened caulk. Finally, use rubbing alcohol or nail polish remover to wipe off residue, and then clean the surface with a sponge and water.
Get the gunk out
Flush out sediment
Working on the water lines always shakes sediment loose. The last thing you want is for these deposits to clog your new faucet. Avoid this problem by purging the lines before hooking up the new supply lines.
Once the entire project is complete and the new supply lines are attached to the faucet, fasten the old supply lines to the shutoff valves. Next, turn the water all the way on for a full minute to wash away any debris in the lines. Then attach the new lines to the shutoff valves. After three days, take the aerator off the faucet and rinse away any sediment that has seeped through.
Make sure you have enough holes
Match faucet to sink openings
Most sinks have three holes for the faucet and a fourth for an accessory, such as a sprayer or a soap dispenser. But some faucets require only one or two holes, and you may not want enough accessories to use the rest. You can buy plugs for unused holes, but they usually don't match the sink. If the sink doesn't have enough holes, cutting an extra hole in stainless steel or cast iron is often difficult or impossible.
To avoid these hassles, choose the faucet and accessories first, then buy a sink with a matching number of holes. Some sinks have “knockouts” that you can drill to provide extra holes. You can also special-order a sink with the number of holes you need.
Trash all the old parts
Replace the entire waste assembly
Resist the temptation to save a few bucks by reusing the old drainpipes. The threads are probably corroded and won't form a tight connection. A new drain assembly is easier to install and less likely to leak.
Instead of shopping for each individual part for the drain, buy a kit at a home center that has everything you need. A sink kit includes drainpipes, fittings, shutoff valves, supply lines and new basket strainers.
Test the shutoff valves first
Replace leaky valves
If the shutoff valves under your sink don't work or you don't have any, you'll have to turn off the water supply to the entire house while replacing the sink. This could cause domestic strife, especially if the job turns into a half day or longer project, so make sure the valves work before going to the home center.
To test the valves, close them and turn on the faucet. The faucet may drip for a minute or two, but if the drip continues, the shutoff valves are leaking. Repair or replace old valves. If you're buying new ones, use quarter-turn balltype shutoff valves. They're more reliable and less likely to leak at the packing nut.
Rusty old pipe? Cheat!
Solve tough problems with couplings
Corroded steel drainpipes are a bear to work with, since the slip nuts are almost impossible to loosen or retighten. You can easily bypass those rusty old threads by adding a section of plastic pipe.
If the slip nut attached to the drainpipe in the wall won't come off, spray on WD-40 and try a bigger wrench. If that doesn't work, cut off the drainpipe with a hacksaw (save as much of the threaded area as possible). Then buy a plastic trap adapter, a transition coupling and a piece of plastic pipe (PVC or ABS) and cement (sold at home centers). Cement the adapter to a 4-in. section of pipe, then place the coupler over the other end of the pipe and over the steel drainpipe.