The spiral threads on these USP Twist Loc Flush Contacts (5/8" Gap, NC, 3/8" Stubby Fit) make installation easy on doors, windows, or other openings. It only takes one turn to install, and one turn to remove for tests. Lightning resistance and 100-million life cycle operation make these contacts ideal for long-term optimum performance. Contacts are UL Listed and approved by the California Fire Marshal for use with fire doors.