Every common metric and SAE hex key with decimal equivalents, the key that fits each socket head cap screw — and whether a cross-system swap is safe or will round the socket.
Be stricter with hex keys than with sockets: an internal hex rounds out much more easily than a bolt head. If the fit has any wobble, use the correct system's key.
Socket head cap screws (the common Allen bolt). Metric screws take metric keys, inch screws take SAE keys — the pairing is standardized.
| Screw size | Hex key | Key (decimal) |
|---|
Standard socket head cap screws per ANSI/ISO. Set screws and button/flat heads often take one size smaller — check before you lean on it.
| Key | System | Decimal inch | mm |
|---|
5 mm for a standard M6 socket head cap screw. The metric run is easy to remember: M4→3, M5→4, M6→5, M8→6, M10→8, M12→10.
Only for the near-twins: 2 mm ≈ 5/64" and 8 mm ≈ 5/16" are close enough for careful use. Most other pairs are loose enough to round the socket — and a rounded internal hex is much harder to rescue than a rounded bolt head.
Usually a worn or slightly-undersized key (often the wrong system), a ball-end used at high torque, or the key not fully seated. Push the key all the way in and use the straight end for breaking loose.