Coupling nuts, also known as extension nuts, are threaded fasteners used for joining two male threaded rods. The exterior of this fastener is typically a hex so a wrench can hold it easily. We offer a variety of stainless steel coupling nuts in a range of sizes.Rod Coupling Nuts are used to join two externally threaded objects, usually threaded rod, together. Reducing Coupling Nuts are available to couple two or more studs of different diameters together.