spud

A spud is a short fastener with threads on both ends and a hexagonal body between them. The two threaded ends screw into different components, creating a rigid connection where conventional bolts cannot fit due to space constraints. In plumbing, spuds typically measure between 0.5 and 2 inches in total length, with the unthreaded hex section occupying roughly the middle third. They are most commonly made from brass or stainless steel to resist corrosion from water and mineral deposits.

Spuds excel where you need to join two female-threaded ports on adjacent fittings with minimal clearance. A toilet tank to flush valve connection, a water heater inlet to an isolation valve, or a sink faucet base to the supply stub are typical applications. Because both ends thread into fixed positions, there is no need for a nut on the opposite side, and the hex body can be gripped with a wrench to control tightness and prevent rotation of the components being joined.

Variants and grip points

Spuds come in various thread pitches and sizes, commonly 1/2 inch and 3/4 inch NPT (National Pipe Thread) for domestic plumbing. Some spuds feature a shoulder or flange at one end, widening the hex body to accommodate fittings that sit closer together. The hex body width typically ranges from 0.75 to 1.25 inches, sized to accept a standard adjustable wrench or spud wrench, a purpose-built tool with a jaw designed specifically to grip the narrow hexagonal profile without slipping.

The main failure mode is over-tightening, which strips the internal threads of the softer fitting (often a brass body) while leaving the harder steel spud undamaged. Once threads are stripped, the connection leaks and must be replaced. Back-off torque, where vibration gradually loosens the spud, also occurs if threads are not properly sealed with PTFE tape or pipe dope. Corrosion at the thread interface, particularly where dissimilar metals meet, can lock a spud in place and make removal difficult without damaging the fittings.

The term 'spud' likely derives from its resemblance to a short, stubby tool, though the plumbing trade etymology is not universally documented. In other trades, 'spud' refers to a digging or prying tool entirely unrelated to fasteners. Within plumbing, the word is unambiguous and widely recognized among installers and supply houses across North America, though British and European plumbers may refer to the same component as a 'double-ended nipple' or 'connecting nipple'.