A coupling nut, also known as extension nut, is a threaded fastener for joining two male threads, most commonly a threaded rod, but also pipes. The outside of the fastener is usually a hex so a wrench can hold it. Variations include reducing coupling nuts, for joining two different size threads; sight hole coupling nuts, which have a sight hole for observing the amount of engagement; and coupling nuts with left-handed threads. These are used to make up long rod assemblies from shorter lengths of rods. The rods are threaded into the coupling nut for a certain distance. Coupling nut dimensions are described by the Industrial Fasteners Institute in standard IFI-128. It is given a plain finish. It is either cold drawn or hot rolled depending on the bar stock used. Coupling nuts threaded with two different thread sizes and profiles are called reducing coupling nuts.
How to use a coupling nut? Coupling nuts can be used to tighten a rod assembly inward or to press a rod assembly outward. You can make a driver tool for hanger bolts using a coupling nut and a hex socket head shoulder screw. Cut off the screw's head, thread the screw into the nut, insert the shoulder into a drill/driver and thread the hanger bolt in the other end of the coupling nut.
Coupling nut size chart