The coupling nut, also known as extension nut, is the threaded fastener for joining two male threads.They are different from other nuts because they are long internally threaded nuts designed to join two male threads together through providing an extended connection.They most commonly used with threaded rod, but also pipes. The outside of the nut is usually a hex so a wrench can hold it. Sizes: Metric sizes range from M4-M36, inch sizes range from 1/4 '' to 2 1/2 ''. Package Type: carton or bag and pallet. Payment terms: T/T, L/C. Delivery Time: 30 days for one container. Trade Term: EXW, FOB, CIF, CFR.