Abstract: A fastener assembly for use in locking relationship with an internally threaded member is disclosed which includes an expandable bolt having a cavity extending longitudinally therethrough which is cylindrically shaped on a first end segment thereof and frusto-conically shaped on a second adjoining end segment thereof. The assembly also includes an elongated draw rod having a cylindrically shaped end portion adapted for close fitting slidable engagement in the first end segment, and a frusto-conically shaped end portion or boss having a rate of taper which is greater than the rate of taper of the second cavity segment. The bolthead defines a first circularly shaped recess concentrically formed about an opening of the cavity and a larger diameter second circularly shaped recess essentrically formed around the first recess.