Spud: potato or threaded fastener nut

To the grocer and the cook, a spud is a potato. To a plumber, a spud is a short nut threaded on both ends, designed to join two parts that need to be pulled together firmly and sealed. The spud nut sits between the toilet tank and the bowl, clamping them together while a rubber washer (called a spud washer) sits atop it to absorb vibration and prevent leaks.

The plumbing spud is typically made of brass or stainless steel and is roughly hexagonal in shape so a wrench can grip it. One end receives a bolt from inside the tank; the other receives a bolt from below the bowl. As you tighten both sides, the nut stays fixed in the middle, drawing the two ceramic or porcelain surfaces together with even pressure. The threads must be precise: too loose and water seeps out; too tight and you crack the tank.

The spud washer that accompanies it is equally important. This flanged ring of rubber or foam sits directly under the spud nut on the bowl side. It cushions the joint, absorbs minor impacts from tank refill cycles, and distributes the clamping force across the bowl's fragile rim rather than concentrating it in one spot.

Why 'spud' for this fastener remains unclear in written sources, though the term is well established in plumbing trade manuals and supplier catalogs. The word may be a corruption or repurposing of an older term, or it may reflect regional trade slang that outlasted its original meaning. What is certain is that every toilet installed by a professional plumber relies on at least one spud nut to stay watertight.