Naval Maintenance and Mechanical Tools – Technical Manuals, Fasteners, and System Operations Practice Test

In US Navy maintenance practice, fastener sizes are stated using U.S. customary units. The minimum nominal diameter for Level I fasteners is given in inches because inch-based sizing underpins the standard fastener catalogs and thread charts used for Level I hardware. Metric units (millimeters) are used when a metric standard is specified, but not for these Level I fasteners. Feet are not used for a diameter measure. The nominal diameter refers to the standard size designation of the threaded portion, not the exact measured value, and inches provide the widely adopted reference for this context.