Installing a toilet tank onto a toilet bowl is a 30-minute job that requires no plumbing experience and one critical skill: knowing when to stop tightening. The tank is porcelain. Overtightening the bolts that hold the tank to the bowl cracks the porcelain. A cracked tank or bowl cannot be repaired. The entire toilet must be replaced. The correct tightness is snug, not tight. Here is how to assemble the tank, install the gaskets correctly, and tighten the bolts enough to prevent leaks without destroying the toilet.
The Three Sealing Points Between Tank and Bowl
Three components seal the connection between the tank and the bowl. Each must be installed correctly or the toilet will leak. The tank-to-bowl gasket, also called the spud washer, is a large rubber or foam gasket, typically 2 to 3 inches in diameter, that fits over the flush valve outlet on the bottom of the tank. It seals the main water passage between the tank and the bowl. This gasket must be centered on the flush valve and must be compressed evenly when the tank is tightened down. The tank bolts run through holes in the bottom of the tank and through corresponding holes in the bowl. Each bolt has a rubber washer inside the tank and a metal washer and nut underneath the bowl. The rubber washer seals the bolt hole inside the tank. The nut holds the tank to the bowl. The rubber washers must be inside the tank, under the bolt heads. The metal washers and nuts go underneath the bowl.
A standard toilet has two tank bolts. Some have three. The bolts, washers, and nuts are included with a new toilet. If you are replacing the original parts, buy a tank-to-bowl bolt kit, which costs $5 to $10 and includes the bolts, rubber washers, metal washers, and nuts.
Assemble the Tank Internals Before Mounting
Install the flush valve and the fill valve in the tank before mounting the tank on the bowl. The flush valve is the assembly in the center of the tank that releases water into the bowl when you flush. The fill valve is the assembly on the left side that refills the tank. Both are easier to install with the tank upside down on a towel on the floor than with the tank mounted on the bowl.
The flush valve installs through the large hole in the center of the tank bottom. The flush valve gasket goes inside the tank. The lock nut threads onto the flush valve body from outside the tank. Tighten the lock nut by hand, then an additional quarter turn with a wrench. The flush valve must be vertical and centered. The fill valve installs through the hole on the left side of the tank. The fill valve gasket goes inside the tank. The lock nut threads on from outside. Tighten by hand. Connect the refill tube from the fill valve to the overflow tube on the flush valve. The tube directs a small stream of water into the overflow tube to refill the bowl after a flush. The tube clips to the top of the overflow tube. It must not extend below the water line in the tank, which would create a siphon that drains the tank continuously.
Install the Tank-to-Bowl Gasket
Place the large tank-to-bowl gasket over the threaded outlet of the flush valve on the bottom of the tank. The gasket presses onto the outlet and stays in place by friction. The tapered side of the gasket faces down toward the bowl. The flat side faces up toward the tank. A correctly installed gasket sits flush against the bottom of the tank and covers the flush valve outlet completely.
If the gasket is foam, it may be stored compressed in the packaging. Let it expand for a few minutes before installing. A foam gasket that is still compressed does not seal properly. If the gasket is rubber, check that it is pliable, not cracked or hardened. A gasket that has been sitting on a hardware store shelf for years may be past its useful life. A new gasket is included with a new toilet. If you are reinstalling an old tank, buy a new gasket. The gasket is cheap. The water damage from a leak is not.
Install the Tank Bolts
Insert the tank bolts through the holes in the bottom of the tank from the inside. The bolt head goes inside the tank. The threaded end extends down through the bottom. Place a rubber washer under each bolt head inside the tank. The rubber washer seals the bolt hole. Do not place a metal washer inside the tank. The metal washer goes under the nut outside the bowl. The metal washer inside the tank will not seal against water and will rust.
A thin bead of silicone caulk under each rubber washer inside the tank provides an additional seal. This is optional but recommended, particularly if the tank is older and the porcelain around the bolt holes is rough or chipped. The silicone fills small irregularities in the porcelain and ensures a watertight seal. Apply the silicone, place the rubber washer, and insert the bolt. Tighten the bolt just enough to compress the rubber washer slightly. The final tightening happens after the tank is on the bowl.
Mount the Tank on the Bowl
Lift the tank and position it over the bowl. The bolts protruding from the bottom of the tank must align with the holes in the bowl. Lower the tank slowly. The large gasket must seat squarely onto the bowl inlet. The gasket should contact the bowl evenly all the way around. If the gasket is not centered, lift the tank, reposition the gasket, and lower it again. Do not slide the tank sideways once the gasket contacts the bowl. Sliding unseats the gasket.
Thread a metal washer and a nut onto each bolt from underneath the bowl. Tighten the nuts by hand. Then tighten each nut with a wrench a quarter turn at a time, alternating between the left and right bolts. The goal is to compress the large gasket evenly and draw the tank down until it is level and stable. The tank should sit level on the bowl with no rocking. The gap between the tank and the bowl should be even on both sides and front to back.
Stop tightening when the tank is snug and does not rock. The correct tightness is approximately hand-tight plus one to one-and-a-half turns with a wrench. You should not need to lean into the wrench. If you do, you are overtightening. A tank that is tightened too much will crack at the bolt holes, either immediately or weeks later when the stress crack propagates through the porcelain. The crack starts at the bolt hole and runs outward. Water leaks from the crack, not from the seal. The tank must be replaced.
Connect the Water Supply and Test
Connect the water supply line to the fill valve. Tighten the nut by hand. Turn the water on slowly at the shutoff valve. Watch the tank-to-bowl connection as the tank fills. Check for water at the bolt heads inside the tank and at the nuts underneath the bowl. Check the large gasket for leaks. A few drops of water that stop when you tighten the nuts slightly are normal. A steady drip that continues after tightening indicates the gasket is not seated correctly or a rubber washer is pinched.
Flush the toilet. The flush sends a surge of water through the gasket and tests the seal under dynamic pressure. Check for leaks again during and after the flush. If the connection is dry, the installation is complete. Install the bolt caps over the nuts under the bowl. Install the tank lid.
Frequently Asked Questions
How tight is tight enough? I am afraid of cracking the tank.
Tighten until the tank does not rock and the nuts offer firm resistance to the wrench. This is typically hand-tight plus one full turn. The rubber washers and the large gasket create the seal through compression, not through extreme bolt tension. If water leaks at this level of tightness, the gasket is not seated correctly or a rubber washer is defective. Tightening further to stop a leak masks the real problem and risks cracking the porcelain. If the connection leaks at the correct tightness, remove the tank, inspect the gaskets and washers, and reinstall.
I am assembling a two-piece toilet for the first time. Is anything different?
No. The process is the same as replacing a tank on an existing bowl. The only difference is that the bowl is not yet bolted to the floor. Install the tank on the bowl before bolting the bowl to the floor. The assembled toilet is heavy, typically 80 to 100 pounds, and is easier to position on the floor as a single unit than to attach the tank to a bowl that is already bolted down. Set the wax ring on the flange, lift the fully assembled toilet onto the flange bolts, and tighten the floor bolts. The tank installation is the same regardless of whether the bowl is on the floor or on its side in the garage.
The large gasket fell off while I was lowering the tank. Do I have to start over?
Lift the tank straight up. Retrieve the gasket. Press it back onto the flush valve outlet. Lower the tank again. The gasket should stay on by friction. If it continues to fall off, the gasket is stretched or is the wrong size. A new gasket costs $5 to $8. Buy a new one. Do not glue the gasket in place. Adhesive prevents the gasket from seating properly and introduces chemicals that can degrade the rubber. A properly sized gasket stays on by friction alone.
Last modified: June 15, 2026
