Abstract: The disclosed method and apparatus is adapted to attach a self-attaching fastener, such as a pierce or clinch nut or stud to a metal wall of an enclosed structure, such as a tube, from inside the structure. The method includes rigidly supporting the self-attaching fastener within the enclosed structure on an anvil with the fastening portion extending toward and located adjacent an inner surface of the wall. A die member is then driven against the outer surface of the wall to deform the wall into locking engagement with the self-attaching fastener. Where a pierce nut or stud is used, the nut simultaneously pierces the wall from inside the structure and clinches the wall as the die member is driven against the outer surface of the wall. The apparatus includes an expandable anvil assembly having opposed surfaces which conform generally to the opposed surfaces of the enclosed structure and a pocket which receives and rigidly supports the body portion of the self-attaching fastener.