Toilet Leaks Between the Tank and the Bowl

When I flush the toilet, I noticed that water seems to leak on the floor from between the tank and the bowl. How can I repair this?

This is symptomatic of a leak in the tank-to-bowl seal or around the tank-to-bowl mounting bolts. Follow the following steps for a quick and sure repair:

Shut off the water to the toilet.
Flush the toilet, and then sponge out the water remaining in the tank. Important: Be sure to take out any bricks, bottles, small water craft, or fish that may remain in the tank.
Disconnect the inlet tube at the toilet, not at the shutoff. If you disconnect the tube at the shutoff compression fitting, there is less of a chance that you will be able to reuse the line later. Look at the line itself. If it is chromed and has a washer on the top, you should try to get a replacement washer when you go to the hardware store. All stores don't have them, so you may want to purchase a new supply tube while you are out. Since there are a few different variations on the supply lines, see How do I install a new intake line to my toilet?
Now comes the fun part. You must now unbolt the toilet tank from the toilet bowl.

If you look inside the toilet tank, you will see two very large slotted screw heads. These go through the tank and the bowl, with a nut on the end under the toilet. You must remove these screws. Sometimes they are easy, and sometimes not. If they do not budge at all, you can drill the screw head out. Using an 1/8" or thereabouts bit, drill one hole through the center of each screw, at least 1/4" in. This is your guide.

Go through your box of drill bits, increasing the size of the hole until the head of the screw comes off. It's made of brass, so the drilling won't be difficult if your bits are sharp! And, since you are working around porcelain, control your drill and be careful not to damage the toilet.
Lift the tank off of the bowl. You will see a disgusting mess of melted rubber and other vile stuff... this was the tank-to-bowl seal! Peel it off the tank, put it in a bag, and drive it over to the undertaker... I mean hardware store. Most hardware stores worth their salt will offer a few different sizes of this seal, so it pays to have something to compare them to. Plus, it gives you something to show off to the guys! Guys like this stuff!
Hey, I didn't say to leave the hardware store yet! You also need to purchase a new tank-to-bowl bolt kit. Even if you didn't have to drill out the old set, don't reuse it. The new kit will supply you all the parts you need, and is designed to make it easier to get a leakproof seal.
The generic tank-to-bowl bolt kits I have used a specific order of assembly. Then are different from the factory kits in that they have additional parts to get a better seal. If you do not assemble them correctly, the seals between the tank and the bolt heads will leak. A metal washer and a fiber washer are put on the bolt. Slide the bolt into the tank holes from the inside. The fiber washer should be in contact with the inside of the tank. There is a nut that must be secured to the bottom of the tank. This nut tightens the bolt head against the inside of the tank, squeezing the fiber washer tightly to form a leakproof seal.
Place the new tank-to-bowl seal on the base of the flapper assembly under the tank. If your water is heavily chlorinated, rubbing some silicone plumber's grease onto the seal should increase its useful life. Don't use Vaseline petroleum jelly... it may react with the rubber and shorten its life!
Position the tank over the bowl so that the bolts go through the holes in the bowl. Use one fiber washer, one metal washer, and a nut (or in some cases a wing nut) and begin to tighten the tank to the bowl. To prevent breakage of the tank, tighten the nuts a few turns each, keeping even downward pressure on the tank-to-bowl seal.
DO NOT ALLOW THE TANK BOLTS TO TURN WHEN TIGHTENING THE TANK-TO-BOWL NUTS! The bolt heads should be held stationery (using a screwdriver) when tightening the tank-to-bowl mounting nuts. Allowing the bolt head to turn may twist the washer under the bolt head, leading to a leak. (Note: If you are using a manufacturer replacement kit you may only have a single nut per bolt. It is even more important to do all the tightening at the nut, not the bolt, for the same reason... possible damage to the seal.)
Check the orientation of the tank with respect to the intake line. If you are going to reuse the old intake line, it should be close enough to the threads on the inlet valve to begin threading the coupling nut. You can start threading the coupling nut, but do not tighten it completely at this point.
Continue to alternately tighten the nut onto the tank bolts until the bottom of the toilet tank hits or nearly hits the toilet bowl. The tank should be very solidly placed. Give it a little shake to see. No... the tank!
Before turning on the water supply, pour a gallon of water into the tank to check for leaks. Leaks around the bolts will show up immediately. It is helpful to use a flashlight for this step. Now, flush the toilet and look for leaks around the tank-to-bowl seal.
Finish tightening down the inlet valve coupling nut. Turn on the water slowly and fill the tank and flush. No leaks? Good work!
Siesta time! And not on the bathroom floor... get out of there already!

You may ask... why should I replace the tank-to-bowl gasket when the leak is from one of the bolts? Because I say so! If the toilet is more than a few years old, the gasket has become somewhat stiff. Disturbing it in any way (aside from loud noises... common in many bathrooms) will almost definitely cause a leak. Why take the chance... do it right the first time!!

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