Abstract: A blind fastener for clamping sheet components and including a mechanical lock to prevent loosening of the fastener. The fastener is self-expanding following insertion and engagement of the threaded stem. The fastener includes an internally threaded nut body, an externally threaded stem threadably received within the nut body, and a lock sleeve matingly receiving the nut body. Carried on the stem is an expandable sleeve which expands to engage the underside of the material. Rotation of the stem following insertion of the assembly into the aperture will draw the stem head and expandable sleeve against the backside for the material causing the sleeve to expand outwardly. In addition, the locking sleeve is pushed beneath the expansion sleeve wedging between the threaded stem and expansion sleeve. This mechanical lock is resistant to vibration locking the fastener against loosening.