Hex key size, key depth and point sizes for inch socket set screws, #0 to 1-1/2″. ASME B18.3.
The numbers behind a socket set screw (“grub screw” or “Allen set screw”) — a headless screw with a hex socket in one end and a point on the other. J is the hex key (Allen wrench) that fits it; T is how deep the key goes in; C is the cup or flat point diameter; P and Q are the half-dog point’s diameter and length; the last column is the length at which a cone point becomes 90°. The length of a set screw is its whole length, end to end. Inches throughout.
Letters match the table. Socket shown dashed.
| Size | Key J | Depth T min | Cup/flat C max | C min | Half-dog P max | Q max | Cone 90° from |
|---|---|---|---|---|---|---|---|
| #0 | .028″ | 0.050 | 0.033 | 0.027 | – | – | – |
| #1 | .035″ | 0.060 | 0.040 | 0.033 | – | – | – |
| #2 | .035″ | 0.060 | 0.047 | 0.039 | – | – | 0.13″ |
| #3 | .050″ | 0.070 | 0.054 | 0.045 | – | – | 0.13″ |
| #4 | .050″ | 0.070 | 0.061 | 0.051 | 0.075 | 0.030 | 0.19″ |
| #5 | 1/16″ | 0.080 | 0.067 | 0.057 | 0.083 | 0.033 | 0.19″ |
| #6 | 1/16″ | 0.080 | 0.074 | 0.064 | 0.092 | 0.038 | 0.19″ |
| #8 | 5/64″ | 0.090 | 0.087 | 0.076 | 0.109 | 0.043 | 0.25″ |
| #10 | 3/32″ | 0.100 | 0.102 | 0.088 | 0.127 | 0.049 | 0.25″ |
| 1/4″ | 1/8″ | 0.125 | 0.132 | 0.118 | 0.156 | 0.067 | 0.31″ |
| 5/16″ | 5/32″ | 0.156 | 0.172 | 0.156 | 0.203 | 0.082 | 0.38″ |
| 3/8″ | 3/16″ | 0.188 | 0.212 | 0.194 | 0.250 | 0.099 | 0.44″ |
| 7/16″ | 7/32″ | 0.219 | 0.252 | 0.232 | 0.297 | 0.114 | 0.50″ |
| 1/2″ | 1/4″ | 0.250 | 0.291 | 0.270 | 0.344 | 0.130 | 0.57″ |
| 5/8″ | 5/16″ | 0.312 | 0.371 | 0.347 | 0.469 | 0.164 | 0.75″ |
| 3/4″ | 3/8″ | 0.375 | 0.450 | 0.425 | 0.562 | 0.196 | 0.88″ |
| 7/8″ | 1/2″ | 0.500 | 0.530 | 0.502 | – | – | – |
| 1″ | 9/16″ | 0.562 | 0.609 | 0.579 | – | – | – |
| 1-1/8″ | 9/16″ | 0.562 | 0.689 | 0.655 | – | – | – |
| 1-1/4″ | 5/8″ | 0.625 | 0.767 | 0.733 | – | – | – |
| 1-3/8″ | 5/8″ | 0.625 | 0.848 | 0.808 | – | – | – |
| 1-1/2″ | 3/4″ | 0.750 | 0.926 | 0.886 | – | – | – |
Key depth T applies to the longer screws of each size — very short set screws have a shallower socket. Cone points are 90° at the length shown and longer, 118° on shorter screws. Knurled cup points are cup points. Thread sizes and pitches are on the thread pitch chart.
Which point to use
| Point | Use it for |
|---|---|
| Cup | The most common point — general holding of collars, pulleys and gears on a shaft. The cup edge cuts into the shaft. |
| Flat | Parts that are re-set often — little or no damage to what it bears on. Works on hardened shafts, best against a ground flat. |
| Oval | Frequent adjustment with the least marking, and bearing on angled or grooved surfaces. |
| Cone | Permanent setting — deepest bite and the most holding power. On shafts harder than Rockwell C15, spot-drill the shaft half the point’s depth. Also used as a pivot. |
| Half-dog | Permanent location — the point sits in a hole drilled in the shaft, or against a milled flat. Often used in place of a dowel pin. |
Strength: set screws work in compression, so they are rated by hardness, not tensile strength. Alloy-steel socket set screws are heat treated to Rockwell C45–53 (ASTM F912).
Quick answers
- What size Allen wrench fits a 1/4" set screw?
- A 1/4″ socket set screw takes a 1/8″ hex key (Allen wrench).
- What hex key goes with each set screw size?
- #0 – .028″, #1 – .035″, #2 – .035″, #3 – .050″, #4 – .050″, #5 – 1/16″, #6 – 1/16″, #8 – 5/64″, #10 – 3/32″, 1/4″ – 1/8″, 5/16″ – 5/32″, 3/8″ – 3/16″, 7/16″ – 7/32″, 1/2″ – 1/4″, 5/8″ – 5/16″, 3/4″ – 3/8″, 7/8″ – 1/2″, 1″ – 9/16″, 1-1/8″ – 9/16″, 1-1/4″ – 5/8″, 1-3/8″ – 5/8″, 1-1/2″ – 3/4″.
- How do you measure a set screw?
- End to end — the whole screw. A set screw has no head, so the socket end and the point are both part of the length.
- Which set screw point should I use?
- Cup is the most common, for general holding on a shaft. Use flat for parts that are re-set often, cone for permanent setting and the most holding power, and half-dog to locate in a drilled hole (often in place of a dowel pin).
- How hard are alloy steel set screws?
- Rockwell C45–53 (ASTM F912). Set screws work in compression, so they are rated by hardness, not tensile strength.