What You're Actually Replacing

The "guts" of a toilet are the internal parts that fill the tank with water and let it drain when you flush. The main pieces are the fill valve (also called the ballcock), which lets water in, and the flush valve, which sits at the bottom and lets water out into the bowl. Most people replace these as a kit because they wear out together after five to ten years, causing slow fills, running tanks, or weak flushes.

You do not need a plumber for this job. The parts cost $20 to $60 depending on the kit, and the work takes 30 to 45 minutes if you have never done it before. You will need an adjustable wrench, a flathead screwdriver, and a bucket or towel to catch water.

Key Takeaways

  • Turn off the water supply valve behind or below the toilet before you start, then flush to empty the tank.
  • Disconnect the water line from the fill valve using an adjustable wrench, and unbolt the fill valve and flush valve from underneath the tank.
  • Install the new fill valve first by threading it through the tank hole and tightening the locknut from below, then connect the water line.
  • Install the new flush valve by threading it through the center hole and tightening the locknut, then reattach the flapper and overflow tube.
  • Turn the water back on, let the tank fill, and adjust the fill valve float so the water stops about one inch below the overflow tube.

Turn Off Water and Empty the Tank

Locate the water supply valve on the wall or floor behind the toilet. It is a small knob or lever. Turn it clockwise (or flip the lever perpendicular to the pipe) until it stops. You should feel resistance but do not force it.

Flush the toilet once. The tank will empty and the fill valve will not refill it because the water is now shut off. If water is still trickling in, the shutoff valve is leaking and you may need to turn it harder or replace it later. For now, use a sponge or towel to soak up any remaining water in the tank so you have a dry workspace.

Remove the Old Fill Valve

Look under the tank where the water line enters. You will see a nut holding the fill valve in place. Use your adjustable wrench to hold that nut while you unscrew the compression nut on the water line itself (the one connecting the line to the fill valve). Water may drip—this is normal. Have your bucket or towel ready.

Once the water line is free, use the wrench to unscrew the large locknut under the tank that holds the fill valve in place. The fill valve will come out from the top of the tank. Set it aside. If the rubber washer under the locknut is cracked or hard, replace it with the new one that comes in your kit.

Remove the Old Flush Valve

The flush valve sits in the center bottom of the tank. You will see a large nut underneath it. Use your adjustable wrench to unscrew this locknut. The flush valve will lift out from the top. As you remove it, the flapper (the rubber seal that lets water into the bowl) will come with it.

Look at the rubber gasket under the flush valve locknut. If it is cracked, hardened, or visibly worn, the new kit will include a replacement. Wipe the area around the hole clean with a dry cloth so the new valve seals properly.

Install the New Fill Valve

Thread the new fill valve down through the hole on the side of the tank where the old one came out. From underneath, slide the rubber washer onto the valve stem, then hand-tighten the locknut. Use your wrench to snug it firmly—tight enough that it does not move, but not so hard that you crack the tank. Most people tighten until they feel solid resistance, then give it a quarter turn more.

Reconnect the water line to the fill valve. The compression nut should thread on by hand first, then you tighten it with the wrench. Hold the nut under the tank with one wrench while you turn the compression nut with the other. Tighten until snug; you do not need to force it.

Install the New Flush Valve

Thread the new flush valve down through the center hole at the bottom of the tank. From underneath, slide the rubber gasket onto the valve stem, then hand-tighten the locknut. Tighten with your wrench the same way you did the fill valve—snug and firm, not forced.

From the top of the tank, slide the new flapper onto the overflow tube (the hollow pipe that stands up in the center of the tank). The flapper should hang freely and seal against the flush valve opening when it rests. Attach the refill tube—a small plastic tube that comes with the kit—to the overflow tube so it directs water into the bowl during the refill cycle.

Refill and Adjust

Turn the water supply valve back on by turning it counterclockwise (or flipping the lever parallel to the pipe). The tank will begin to fill. Watch the fill valve and listen for the water to stop. It should stop when the float (a ball or cup attached to the fill valve) rises to shut off the water.

The water level should be about one inch below the top of the overflow tube. If the water level is too high, the fill valve will not shut off completely and water will run into the overflow tube and down into the bowl. Adjust the float by bending the float arm slightly downward (if the water is too high) or upward (if it is too low). Make small adjustments—a quarter inch of arm movement changes the water level by about half an inch. Test by flushing and watching the refill cycle two or three times.

Frequently Asked Questions

What if water is still running into the bowl after I finish?

The flapper is not sealing properly. Check that it is sitting flat against the flush valve opening and that no debris is stuck under it. If the rubber is cracked or warped, you may have received a defective flapper and should replace it. If the flush valve itself is cracked, you will need a new one.

Can I replace just the fill valve or just the flush valve?

Yes. If only one part is failing, you can buy and replace just that part. However, if your toilet is more than seven years old and one part is worn, the other usually is too. Replacing both at once saves you from doing this job again in a year.

How do I know which kit to buy?

Take a photo of the inside of your tank and bring it to a hardware store, or measure the diameter of the holes at the bottom of the tank. Most kits fit standard two-inch holes. Universal kits work in almost all toilets made in the last 20 years, but some older or specialty toilets need specific parts.

What if the locknut will not budge?

explore penetrating oil (like WD-40) and wait 15 minutes. Use a wrench with a longer handle for more leverage, or tap the wrench gently with a hammer to break the corrosion loose. Do not use a pipe wrench on the tank itself—it can crack the porcelain.

Do I need to drain the whole tank before starting?

Flushing empties most of it, but a little water always remains. You do not need to drain it completely. A sponge and bucket are enough to manage the small amount that spills when you disconnect the water line and remove the valves.