Coupling nut

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A coupling nut and its orthographic view

A coupling nut, also known as extension nut, is a threaded fastener for joining two male threads, most commonly a threaded rod,[1][2] but also pipes.[3] 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.[1]

Coupling nuts can be used to tighten a rod assembly inward or to press a rod assembly outward.[4][5]


  1. ^ a b "Coupling Nuts". About Nuts, Materials, and Finishes. McMaster-Carr. Retrieved 26 March 2013. 
  2. ^ "Extension Nut / Coupling Nut". Kisan Engineers Machine Tools. Archived from the original on 27 December 2011. 
  3. ^ American Railway Master Mechanics' Association (1922). Locomotive Cyclopedia of American Practice (sixth ed.). New York: Simmons-Boardman. p. 32. 
  4. ^ Nunn, Richard V. (1982). Popular Mechanics Guide to Do-It-Yourself Materials. New York: Hearst Books. p. 54. ISBN 978-0-87851-150-1. 
  5. ^ Olin, L.; Stanland, A.; Allard, F. (1987). "Fiber Optic Hull Penetrators for Submarines". In O'Donnell-Leach, Karen. Military Applications of Fiber Optics. Fiber Optics Reprint Series Volume 14. Brighton, Massachusetts: Information Gatekeepers Inc. pp. 58–63, page 61.