Allen / Hex Key Size Finder
Pick the right hex key for any socket-head cap screw
किसी भी socket head cap screw के लिए सही allen key
Allen keys (also called hex keys) come in a set of standard sizes designed to match socket-head screws. For ISO-standard socket-head cap screws (Unbrako's default), the rule is: M3 uses 2.5 mm, M4 uses 3 mm, M5 uses 4 mm, M6 uses 5 mm, M8 uses 6 mm, M10 uses 8 mm, M12 uses 10 mm, M16 uses 14 mm, M20 uses 17 mm. These are fixed by ISO 4762 and don't vary between manufacturers. Hex socket set screws (grub screws) use a different set, usually one size smaller: M4 uses 2 mm, M6 uses 3 mm, M8 uses 4 mm, M10 uses 5 mm. Countersunk socket screws (ISO 10642) match cap screws up to M8 but then diverge. This finder covers all three types and warns when ball-end L-keys or T-handles are a better choice for the application.
Calculator
For higher-torque applications, use a T-handle or 3/8"-drive hex bit in a torque wrench. Straight L-keys can slip and round the socket at high torque.
ƒFormula
No formula — ISO 4762 lookup tableAllen key sizes are standardised by ISO 4762 (socket head cap screws) and ISO 4027 (hex set screws). There's no calculation — always refer to the table. Manufacturers like Unbrako, Holo-Krome, and Bossard all follow the same sizes.
Units: Across flats (AF) of hex socket, millimetre (mm)
Allen key ki size kaise decide karein?
Unbrako socket head cap screws (SHCS) ke liye allen key ki size ISO 4762 standard se fix hoti hai, manufacturer change ho to bhi size same rehti hai. Common pairs: M6 cap screw → 5 mm allen, M8 → 6 mm, M10 → 8 mm, M12 → 10 mm, M16 → 14 mm. Set screws (grub screws) usually ek size chhoti key use karte hain: M6 set screw → 3 mm allen, M8 → 4 mm, M10 → 5 mm. Countersunk socket screws ki size cap screw jaisi hi hoti hai chhote sizes mein, lekin bade sizes mein different hai. Ball-end L-keys tight corners mein helpful hain; T-handle keys heavy torque ke liye.
Worked Examples
Assembling a fixture with Unbrako M8 cap screws
Problem: Four M8 × 30 mm Unbrako grade 12.9 cap screws in a bracket.
Solution: M8 SHCS uses a 6 mm allen key. For 42 Nm torque (grade 12.9 dry), use a T-handle 6 mm allen or a 3/8-inch drive hex bit with a torque wrench. Straight L-keys slip on high torque.
Loosening a stuck grub screw on a pulley
Problem: The M6 grub screw locking a pulley onto a shaft is stuck. Which key?
Solution: M6 hex set screw uses a 3 mm allen key. Heat the pulley area gently to break any thread-lock, then use a ball-end 3 mm for corner access. Replace if the socket is rounded.
Countersunk screw confusion
Problem: M10 countersunk socket screw (CSK) — same 8 mm key as the cap screw?
Solution: For ISO 10642 M10 countersunk socket screw, the hex key is 6 mm (not 8 mm like the cap screw). Always verify with the actual screw — countersunk variants have a smaller hex socket.
Allen Key (Hex Key) Size Chart, All Socket Screw Types
Allen key chart — सभी socket screws के लिए
| Bolt Size | Cap Screw (SHCS) | Set Screw / Grub | Countersunk (CSK) |
|---|---|---|---|
| M1.6 | 1.5 mm | 0.7 mm | — |
| M2 | 1.5 mm | 0.9 mm | 1.3 mm |
| M2.5 | 2 mm | 1.3 mm | 1.5 mm |
| M3 | 2.5 mm | 1.5 mm | 2 mm |
| M4 | 3 mm | 2 mm | 2.5 mm |
| M5 | 4 mm | 2.5 mm | 3 mm |
| M6 | 5 mm | 3 mm | 4 mm |
| M8 | 6 mm | 4 mm | 5 mm |
| M10 | 8 mm | 5 mm | 6 mm |
| M12 | 10 mm | 6 mm | 8 mm |
| M14 | 12 mm | 6 mm | 10 mm |
| M16 | 14 mm | 8 mm | 10 mm |
| M18 | 14 mm | — | 12 mm |
| M20 | 17 mm | 10 mm | 12 mm |
| M22 | 17 mm | — | 14 mm |
| M24 | 19 mm | 12 mm | 14 mm |
| M27 | 19 mm | — | — |
| M30 | 22 mm | 14 mm | — |
Sizes are ISO 4762 (SHCS), ISO 4027 (set screws), and ISO 10642 (CSK). Unbrako, Holo-Krome, and all major brands follow these. Ball-end L-keys trade some torque capacity for angle tolerance — use straight keys for high-torque applications.
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