Abstract: A self-locking nut mechanism is operable to be mounted on a bolt element to secure structural members together and includes a nut body having a cylindrical portion and a flange portion. A through hole in the nut body has a first hole portion with one diameter and a second hole portion with another larger diameter, the nut body having a transition section between the first and second hole portions with such transition section having a first tapered part. A split nut is disposed in the second hole portion, the split nut being split into at least two split-nut sections. The split nut has a longitudinal end having a second tapered part. A spring in the nut body biases the split-nut toward the first tapered part such that the first and second tapered parts are biasingly engaged. The bolt element has external fastenings, and the split-nut has internal fastenings.