So, for critical applications, Stage 8 Locking Fasteners, San Rafael, Calif. (stage8.com), developed GrooveLok technology It uses a locking retainer ring that fits into a small milled slot next to the counterbore. The ring acts like a wrench handle braced against a stationary object (such as a frame, casting, protrusion, or side of a counterbored hole) to prevent counterrotation of the bolt. If there is nothing for the ring to brace against, a bridge that fits over two to three bolts can be used, or the ring can be bent over an edge or frame. The system works in temperatures from -200 to 1,000°F and withstands vibrations over 100 Hz. The company even guarantees the fasteners will never loosen, until you disassemble them. The fasteners retrofit on existing machines, are fully reusable, and have been used on everything from eyeglass screws to 8-in. bolts holding generator propellers in hydroelectric dams — without a single reported failure. The system is said to be easy to install, and, once in place, users are assured the system is still working at full load if they see the ring and clip in place.