Short definition
Tank bolts are the two or three brass bolts that secure a tank to a bowl on a two-piece toilet. Each passes through a rubber washer above the porcelain, through the tank floor, and into a brass nut underneath. Together they compress the tank-to-bowl gasket and seal the joint.
What it is
Standard size: 1/4-inch diameter brass, 2-1/4 to 3 inches long. Brass throughout — washers can be rubber-and-metal stacks, but the bolts themselves should always be brass to avoid rusting in the always-damp tank environment.
Tank bolt installation is a cross-tightening operation: alternate sides in quarter-turns to compress the tank-to-bowl gasket evenly. Tightening one side fully before the other distorts the gasket and sets up a slow leak.
A typical tank-bolt kit includes two or three bolts, brass nuts, rubber washers (above the porcelain to prevent the bolt from gouging the tank floor), metal washers (under the brass nuts for load distribution), and the tank-to-bowl gasket itself.
Why it matters to a homeowner
When you have a slow leak between tank and bowl, the first suspect is tank bolts (loose or worn rubber washer); the second is the tank-to-bowl gasket. Both are inexpensive parts ($5 to $20) and the repair takes about an hour with the toilet off the floor.
Always replace tank bolts when you replace the gasket. The rubber washers harden over time, and a $5 fresh bolt set is cheap insurance against another leak in a year.
The classic install mistake — over-torquing — cracks the tank around the bolt holes. Cracked tank means tank replacement (or, more often, whole-toilet replacement). Tighten until snug, then stop.
Common failure modes
- Worn rubber washer at top of bolt. Leak around bolt head inside the tank.
- Cracked porcelain at bolt hole (over-tightening). Tank replacement.
- Corroded brass (rare; brass resists corrosion). Replace.
- Over-torqued one side. Unequal compression on the tank-to-bowl gasket; slow leak.
Common variants
- Tank bolts (this entry, tank-to-bowl) vs. closet bolts (bowl-to-flange). Different connection.
- Tank bolts vs. spud nut. Spud nut is for the flush valve; tank bolts are for the tank-to-bowl connection.