Abstract: A fastener having an internally threaded nut body, an externally threaded stem and a lock collar keyed to the nut body. The lock collar has a plurality of spring arms extending radially at an angle into the internally threaded bore of the nut body. During setting of the fastener, the stem is rotated in a tightening direction relative to the nut body. However, when the fastener is fully set, the spring arms will engage axial grooves in the stem to prevent relative rotation in a loosening direction between the nut body and the stem. This configuration forms a positive mechanical lock between the stem and the nut body and prevents the fastener from loosening and losing its clamping force, even when the fastener is subjected to high levels of stress and vibration.