Some plumbers call the thick rubber gasket between a toilet tank and bowl the "spud gasket," while others call it a "cone gasket." You usually don't have to worry about it, but it may need to be replaced in the rare instance when the tank spews water on the floor under the toilet. The water is usually coming from the bolts holding the tank to the bowl, but it could be coming from the spud gasket. If inspection reveals the spud to be at fault, and you have to replace it, that gives you a chance to service the tank-to-bowl bolts as well.
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Step 1: Disconnect the water supply.
Turn off the water valve and flush the toilet, then sponge all the water in the bottom of the tank into the bowl. Unscrew the water connection from the fill valve on the bottom of the tank, using your fingers or adjustable pliers.
Step 2: Unscrew the tank bolts and remove the tank.
Lock a pair of locking pliers onto the nut underneath the tank with which each bolt is secured. Turn the head of the bolt with a screwdriver -- flat-head for most bolts -- to loosen the nut. The locking pliers will turn with the head until the pliers meet the bowl, at which point the nut will loosen. Unscrew each nut the rest of the way by hand, then remove the nuts and lift the tank off the bowl.
The tank bolts may be too corroded to unscrew. If so, you may have to cut them off from underneath the tank, using a hacksaw, or even drill them out.
Even if you don't see major damage, replacing the tank's bolts and washers with new ones can help you avoid problems later.
Step 3: Replace the spud.
Pull off the old spud and fit an identical replacement around the inlet pipe on the bottom of the tank. Push it up against the bottom of the tank securely to make sure it doesn't move when you replace the tank.
Step 4: Replace the tank.
Set the tank back in place, align the holes in the bottom with the holes in the bowl. Fit a rubber washer around each tank bolt and feed a bolt through each hole. Slide a plastic washer up each bolt and screw on a nut, tightening each nut as much as you can by hand. Hold the nuts with pliers and continue tightening each bolt with a screwdriver. Tighten each one incrementally, then tighten the other by the same amount to keep the tank balanced.
The spud is thick and flexible, so the tank may still move slightly when the bolts are tight. Don't try to make them so tight as to prevent this slight movement, or you could crack the tank.
Step 5: Turn on the water.
Connect the supply hose to the fill valve and turn on the water valve. Watch carefully for leaks -- especially from the tank bolts. If you see water dripping, tighten the bolts a little more.