A threaded fastener has a casing enclosing at least three radially inwardly biased arcuate segments positioned to engage the threads of a bolt or hollow threaded member. The casing defines spaced apart internal frustoconical surfaces positioned to engage corresponding surfaces of the segments in the same axial direction. The radially inner ends of the frustoconical surfaces are directed toward a first end of the fastener, and the diameter of the radially inner end of the frustoconical surface away from the first end of the fastener is less than the radially outer diameter of the other frustoconical surface. The threads of the segments substantially fully engage the threads of the bolt or hollow threaded member.