What is a no. 1 screw

A #1 screw has a major diameter of approximately 0.073 inches (about 1.85 mm) in the numbered screw size system, representing one of the smallest commonly available machine screw sizes. The numbering system for small screws runs from #0 (smallest, approximately 1.5 mm diameter) through #12 (approximately 5.5 mm diameter), with each increment representing approximately 0.013 inches (0.33 mm) diameter increase. #1 screws are used in eyeglasses, small electronics, precision instruments, and delicate assemblies where larger fasteners would be inappropriate. Thread specifications for #1 screws include common variants like #1-64 (64 threads per inch, very fine thread for precision instruments) and #1-72 (72 TPI, ultra-fine for watch-making and micro-assembly). Installing #1 screws requires precision screwdrivers—typically 1.0-1.5 mm blade width—and gentle technique, as overtightening easily strips threads in tapped holes or breaks the screw shaft due to limited cross-sectional area. The metric equivalent would be approximately M2 (2 mm major diameter), though thread forms and pitches differ enough to prevent interchangeability in threaded holes. For applications requiring #1 fasteners, consider whether the assembly environment supports such delicate hardware—vibration, thermal cycling, and routine disassembly can compromise connections using these small fasteners, potentially requiring thread-locking compounds, captive washer designs, or alternative joining methods like adhesives for reliable long-term assembly.