Abstract: A quick insertion fastener has an internally threaded first member (“nut”) adapted to be quickly attached and tightened on an externally threaded second member (“bolt”). The nut is formed with a casing with an internal frusto-conical front surface, a plurality of threaded shell segments contained in the casing having external frusto-conical front surfaces which abut the front surface of the casing, a spring member positioned at a rear part of the casing which elastically holds the rear ends of the shell assembly together, and an end retainer provided at the rear part of the casing having a taper for engaging the rear ends of the shell assembly and displacing them radially apart when the shell assembly is displaced axially toward the end retainer by insertion of the bolt. This allows the bolt threads to move without interference past the internal threads of the shell segments.