🔩 General Hardware
🔩 M3 / M4 (Metric Sizes)
RC hardware is almost entirely metric — "M3" means a 3 mm diameter thread, "M4" means 4 mm.
The second number is length: M3x12 is a 3 mm thread, 12 mm long. Note that the driver size is a separate thing — most M3 screws take a 2.0 or 2.5 mm hex driver depending on head type.