M2 M2.5M3 M4 M5 M6 M8 M10 M12 M14 M16 M18 M20 M22 M24. A2 Stainless Steel Coupling Nut Hexagonal Extension Nut Connecting Rod Fitting Screw Sleeve. Coupling nuts are very similar to hex nuts but are much longer giving it the ability to join two externally threaded fasteners and create an extension.
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