Short definition
A spud nut is the large nut on the underside of a toilet tank that secures the flush-valve assembly to the porcelain. Tightening it compresses the spud washer (a rubber gasket between the flush valve and the tank wall), creating the seal between the tank and the flush-valve assembly.
What it is
Standard spud nut sizes: 2-3/4 inch (residential 2-inch flush valves) and 3-3/8 inch (3-inch flush valves on modern HET toilets). The nut is plastic on most aftermarket flush valves; OEM brass on some Toto and Kohler.
The companion tool is a spud wrench — a wide-jaw specialty wrench for the spud nut. Most flush-valve packages include a stamped plastic or thin metal spud wrench. A 12-inch channel-lock pliers usually substitutes if you don’t have the dedicated wrench.
A separate “spud washer” (terminology varies by manufacturer) sometimes refers to the rubber washer between the flush-valve threads and the bowl on a two-piece toilet — paired vocabulary, not the same component.
Why it matters to a homeowner
You encounter the spud nut whenever you replace a flush valve. After draining the tank, the spud nut comes off from underneath, the flush valve lifts out from above, and you reverse the process with new hardware. The trap: over-tightening cracks the tank around the spud, which is a tank-replacement situation.
If you have a tank-to-bowl leak, the spud washer is one of the suspects (along with tank bolts and the tank-to-bowl gasket). A worn spud washer drips between the tank and the flush-valve assembly.
Common failure modes
- Stripped plastic spud nut on cheap aftermarket flush valves. Replace.
- Cracked tank around the spud (rare; usually from over-tightening). Replace tank or whole toilet.
- Worn spud washer. Leak at the flush-valve-to-tank interface. Replace.
Common variants
- Spud nut (this entry, tank-to-flush-valve) vs. spud washer between flush valve threads and bowl (paired vocabulary).
- Spud wrench vs. basin wrench. Basin wrench is for faucet jamb nuts; spud wrench is for the larger toilet flush-valve nut.