How do I change the washers in the bath tub faucet hot and cold knobs and where are they located exactly?

More times than not, it is the 'hot' that drips. Shut-down the hot water, and see if it stops. If it does, then you have to remove the stem to get to the gasket, which is attatched (or likely now almost/totally) non-existent. Change the stem and the seat behind it.

XenoSapien

How do I change the washers in the bath tub faucet hot and cold knobs and where are they located exactly?
Washer-type faucets work with a rubber or composition washer that closes onto a metal washer seat (Fig. 1). The washer can become hardened, worn or the seat wears, causing the faucet to leak. You can close the faucet tighter to stop the leaking temporarily, but this increases the internal damage to the faucet.


To repair the leak, first turn off the water. If there's a shutoff valve beneath the fixture, turn off the water at that point. Otherwise, turn it off at the main house shutoff valve in the basement, utility room, or crawlspace. Turn off the hot water supply at the water heater.


Take the faucet apart by removing the handle (this may not be necessary on some older faucets). Loosen the Phillips-head screw, which usually is beneath a decorative cap in the center of the handle. The cap either unscrews or snaps off when you pry it with a knife blade. If you must use pliers on decorative faucet parts, pad them with electrical tape or cloth to protect the finish. And take special care with the plastic parts found on many modern faucets. Next, lift or pry the handle off its broached stem. Unscrew the packing nut beneath the handle, exposing the rest of the stem. Remove the stem by rotating it in the "on" direction. It will thread out. Reinstall the handle if you have difficulty turning it (Fig. 1). Clean chips from the faucet cavity, but do not use harsh abrasives or a file.


Examine the stem. If the threads are badly corroded or worn, take it to your retailer and get a new stem to match. Clean the stem if it's dirty.


Now look at the washer, which is located on the lower end of the stem and held in place by a brass screw. If the washer is squeezed flat or has a groove worn in it, replace it–this should stop any dripping. Take the washer with you to your dealer to ensure an exact match in size and style. If the brass screw is damaged, too, replace it with a new brass screw.


The washer seat is located inside the faucet body. You probably can't determine if the washer seat is causing the leak just by looking at it. Any faucet that needs frequent washer replacement obviously has a damaged seat. The seat should either be refaced with a seat-dressing tool. A seat-dressing tool is not costly. Every home with washer-type faucets needs one. Use the tool according to the manufacturer's directions, placing it in the faucet along with the packing nut. Then rotate (Fig. 2) until the seat is smooth, and blow out the chips.


Some washer seats can be unthreaded and replaced. Check the faucet body with a flashlight to see if it has a square or hexagonal hole through its center or is slotted for a screwdriver; if so, it is replaceable. However, if the seat simply has a round hole through its center and no slots, it is not replaceable. In this case, dress it with a seat-dressing tool. To replace it, you'll need a faucet seat wrench, which comes with a combination of square and hex heads to fit most faucet seats. Turn the washer seat counterclockwise to loosen, clockwise to tighten (Fig. 3). Add a little silicone rubber sealant (RTV) or pipe joint compound around the threads of the seat before you install it to make it easier to remove during future repairs.


It's important to install the correct type of faucet washer (Fig. 1, bottom). A swiveling washer (C) is preferable to either (A) or (B). To install washer style (C), file the shoulder off the end of the stem, drill out the threads of the screw hole. Instead of rubbing against the seat as it closes, a swiveling washer closes with a straight-down, frictionless action – this allows it to outlast fixed washers.


Following this seat and washer service, your faucet should be like new. Put the parts back together in the reverse order of taking them apart. Spread a bit of petroleum jelly or silicone grease on the threads of the stem to lubricate the faucet's action.


If the faucet leaks around the stem rather than from the spigot, install new packing. You may want to install one of the newer nylon-covered or graphite-impregnated packings–their lubrication allows the faucet handle to turn more freely. Wrap one turn of this packing around the stem just beneath the packing nut (Fig. 1). Use three complete wraps if you're applying string-type packing. Some stems use O-rings, rather than packing. For these stems, replace the O-ring with a matching one to stop a leak. Hand tighten the packing nut, then tighten it another half-turn.
Good luck, Tom