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Coupling Nuts 101

Have you ever been shoulder-deep in the hood of your car, fighting for every last inch of that socket wrench? Coupling nuts are internally threaded fasteners used to connect two externally threaded rods. Essentially, this simple piece of hardware acts as a revolutionary bolt extension for those times when you need a little more room.


The hex design is the most common type of coupler. Similar to a standard hex bolt or hex nut, it has six flat sides that allow it to be tightened using a wrench. Basic versions are meant to join two rods of equal thread pitch and diameter.

A reduced coupling nut is a variation of the hex design. This joint connects two rods with different thread sizes, ultimately reducing or increasing the size of a bolt head.

The less common eye coupling nut is slightly different, as its entire assembly attaches to a beam or a truss, serving to vertically suspend a threaded rod or bolt.


Most coupling nuts are made of carbon steel, which can be further galvanized or zinc-plated to reduce corrosion. Overall strength, as determined by ASTM, is based on size and material grade, assuming that the threaded distance is equal to or greater than the thread diameter.


Size is determined by the width of the hole, or the externally threaded rod. Common sizes range from 3/8 " to 1 1/14" in diameter. Likewise, pitch refers to the distance between threads, which can be as high as 16 counts per inch. While joint also varies, most common applications settle for 1 to 3 inches.

Seemingly basic, yet surprisingly useful, coupling nuts serve many industrial and household functions. Be sure to order a couple couplers for your box of tools in case you should ever need that extra inch. Call (800) 603-6887 today to learn more about the coupling nuts available at Copper State.