DIN 6334 The coupling nut or extension nut is hexagonal and is used to combine two studs. To lengthen the bolts, the nuts are connected together. Make sure the threaded fasteners are held in place with a wrench. There is a small hole on the side of the nut to let you know the tightness of the screw coupling. The main purpose of the telescopic nut is to increase the size of the rod. It can tighten the connecting rod assembly by using industrial tools to provide inward pressure. The same procedure is used to loosen the component that is under external pressure.