Tooling and hardware
Socket head cap screw dimensions
Inch 1960 series (ASME B18.3) #0 to 1 in and metric (ISO 4762) M1.6 to M30, maximum head sizes. A 1/4-20 has a 0.375 in head, 0.250 in tall, on a 3/16 in hex key.
Technical review pending. Formulas and values are cited. This notice comes down after a machinist review.
Source: ASME B18.3 (socket cap, shoulder and set screws, inch series); ISO 4762 (metric hexagon socket head cap screws); ISO 273 (clearance holes for bolts and screws); Unbrako Socket Products Engineering Guide (inch #0 to #5, M1.6 and M30 head dimensions); ASME B18.3 drill and counterbore sizes and ASME B18.3.1M metric counterbore diameters as published in socket screw maker catalog data (ASTM A574 and A574M).
Basis: head diameter and head height are the maximums the standards allow. Hex key sizes are nominal. Inch clearance drills and counterbore diameters are the ASME B18.3 table for socket head cap screws, not a general machine screw clearance chart. Metric clearance holes are the ISO 273 fine and medium series; metric counterbore diameters follow ASME B18.3.1M. Counterbore depth is not listed.
| Size | Basic major diameter (in) | Head diameter max (in) | Head height max (in) | Hex key (in) |
|---|---|---|---|---|
| #0 | 0.0600 | 0.096 | 0.060 | 0.050 |
| #1 | 0.0730 | 0.118 | 0.073 | 1/16 (0.0625) |
| #2 | 0.0860 | 0.140 | 0.086 | 5/64 (0.0781) |
| #3 | 0.0990 | 0.161 | 0.099 | 5/64 (0.0781) |
| #4 | 0.1120 | 0.183 | 0.112 | 3/32 (0.0938) |
| #5 | 0.1250 | 0.205 | 0.125 | 3/32 (0.0938) |
| #6 | 0.1380 | 0.226 | 0.138 | 7/64 (0.1094) |
| #8 | 0.1640 | 0.270 | 0.164 | 9/64 (0.1406) |
| #10 | 0.1900 | 0.312 | 0.190 | 5/32 (0.1563) |
| 1/4 | 0.2500 | 0.375 | 0.250 | 3/16 (0.1875) |
| 5/16 | 0.3125 | 0.469 | 0.312 | 1/4 (0.2500) |
| 3/8 | 0.3750 | 0.562 | 0.375 | 5/16 (0.3125) |
| 7/16 | 0.4375 | 0.656 | 0.438 | 3/8 (0.3750) |
| 1/2 | 0.5000 | 0.750 | 0.500 | 3/8 (0.3750) |
| 5/8 | 0.6250 | 0.938 | 0.625 | 1/2 (0.5000) |
| 3/4 | 0.7500 | 1.125 | 0.750 | 5/8 (0.6250) |
| 7/8 | 0.8750 | 1.312 | 0.875 | 3/4 (0.7500) |
| 1 | 1.0000 | 1.500 | 1.000 | 3/4 (0.7500) |
Head sizes as published, to 3 places. Hex key shows the fraction and its decimal to 4 places; #0 takes a 0.050 in key.
| Size | Close fit drill | Close fit (in) | Normal fit drill | Normal fit (in) | Counterbore diameter (in) |
|---|---|---|---|---|---|
| #2 | 3/32 | 0.0938 | #36 | 0.1065 | 3/16 (0.1875) |
| #3 | #36 | 0.1065 | #31 | 0.1200 | 7/32 (0.2188) |
| #4 | 1/8 | 0.1250 | #29 | 0.1360 | 7/32 (0.2188) |
| #5 | 9/64 | 0.1406 | #23 | 0.1540 | 1/4 (0.2500) |
| #6 | #23 | 0.1540 | #18 | 0.1695 | 9/32 (0.2813) |
| #8 | #15 | 0.1800 | #10 | 0.1935 | 5/16 (0.3125) |
| #10 | #5 | 0.2055 | #2 | 0.2210 | 3/8 (0.3750) |
| 1/4 | 17/64 | 0.2656 | 9/32 | 0.2813 | 7/16 (0.4375) |
| 5/16 | 21/64 | 0.3281 | 11/32 | 0.3438 | 17/32 (0.5313) |
| 3/8 | 25/64 | 0.3906 | 13/32 | 0.4063 | 5/8 (0.6250) |
| 7/16 | 29/64 | 0.4531 | 15/32 | 0.4688 | 23/32 (0.7188) |
| 1/2 | 33/64 | 0.5156 | 17/32 | 0.5313 | 13/16 (0.8125) |
| 5/8 | 41/64 | 0.6406 | 21/32 | 0.6563 | 1 (1.0000) |
| 3/4 | 49/64 | 0.7656 | 25/32 | 0.7813 | 1 3/16 (1.1875) |
| 7/8 | 57/64 | 0.8906 | 29/32 | 0.9063 | 1 3/8 (1.3750) |
| 1 | 1 1/64 | 1.0156 | 1 1/32 | 1.0313 | 1 5/8 (1.6250) |
From the ASME B18.3 drill and counterbore table. #0 and #1 are not in that table. Counterbore depth is not listed; sink at least the head height.
| Size | Head diameter max (mm) | Head height max (mm) | Hex key (mm) | Clearance hole, fine (mm) | Clearance hole, medium (mm) | Counterbore diameter, ASME B18.3.1M (mm) |
|---|---|---|---|---|---|---|
| M1.6 | 3.00 | 1.60 | 1.5 | Not listed | Not listed | Not listed |
| M2 | 3.80 | 2.00 | 1.5 | Not listed | Not listed | 4.40 |
| M2.5 | 4.50 | 2.50 | 2 | Not listed | Not listed | Not listed |
| M3 | 5.50 | 3.00 | 2.5 | 3.2 | 3.4 | 6.50 |
| M4 | 7.00 | 4.00 | 3 | 4.3 | 4.5 | 8.25 |
| M5 | 8.50 | 5.00 | 4 | 5.3 | 5.5 | 9.75 |
| M6 | 10.00 | 6.00 | 5 | 6.4 | 6.6 | 11.25 |
| M8 | 13.00 | 8.00 | 6 | 8.4 | 9 | 14.25 |
| M10 | 16.00 | 10.00 | 8 | 10.5 | 11 | 17.25 |
| M12 | 18.00 | 12.00 | 10 | 13 | 13.5 | 19.25 |
| M14 | 21.00 | 14.00 | 12 | 15 | 15.5 | Not listed |
| M16 | 24.00 | 16.00 | 14 | 17 | 17.5 | 25.50 |
| M20 | 30.00 | 20.00 | 17 | 21 | 22 | 31.50 |
| M24 | 36.00 | 24.00 | 19 | 25 | 26 | 37.50 |
| M30 | 45.00 | 30.00 | 22 | Not listed | Not listed | Not listed |
Head sizes to 2 places; M30 is the plain head maximum (knurled heads can run larger). Clearance holes from the ISO 273 fine and medium series. Counterbore diameters from ASME B18.3.1M only; other standards differ. Not listed means no value is published here.
How to read the table
Find the screw size, then read across. Head sizes are maximums, so a real head measures a little under. On inch screws from 1/4 in up, the head diameter is 1.5 times the screw diameter and the head is as tall as the screw is wide. Low-head and button head screws use other tables and are not on this page.
Worked check: 1/4-20
A 1/4-20 socket head cap screw, D = 0.250 in.
- Head diameter: 1.5 x 0.250 = 0.375 in, matching the table (0.375 in).
- Head height: equal to D, 0.250 in.
- Hex key: 3/16 in (0.1875 in).
- Clearance: 17/64 drill (0.2656 in) close fit, 9/32 (0.2813 in) normal fit; counterbore 7/16 in (0.4375 in), wider than the 0.375 in head.
Result: 0.375 in head, 0.250 in tall, 3/16 in hex key.
Worked check: M6
An M6 socket head cap screw.
- Head diameter: 10.00 mm maximum.
- Head height: 6.00 mm maximum, equal to the 6 mm thread size.
- Hex key: 5 mm. Clearance hole: 6.4 mm fine, 6.6 mm medium. Counterbore: 11.25 mm (ASME B18.3.1M).
Result: 10 mm head, 6 mm tall, 5 mm hex key.
Shop notes
- Metric clearance holes are the ISO 273 series; inch close and normal fit drills are the ASME B18.3 cap screw table. The drawing governs.
- To sink a head flush, the counterbore must be at least as deep as the head height in the table. Add depth if the drawing calls for the head below the surface.
- Hex key sizes are nominal; inch and metric keys are not interchangeable.
- Metric heads are not 1.5 x D across the range: an M6 head is 10 mm, not 9 mm.
FAQ
What size Allen key fits a 1/4-20 socket head cap screw?
3/16 in (0.1875 in).
What is the head diameter of an M6 socket head cap screw?
10 mm maximum, 6 mm tall, on a 5 mm hex key.
What clearance hole for an M8 cap screw?
8.4 mm for a fine fit, 9 mm for a medium fit (ISO 273).
What clearance drill and counterbore for a 1/4-20 socket head cap screw?
A 17/64 drill (0.2656 in) for a close fit or 9/32 (0.2813 in) for a normal fit, with a 7/16 in (0.4375 in) counterbore, from the ASME B18.3 drill and counterbore table.
What counterbore diameter for an M6 cap screw?
11.25 mm per the ASME B18.3.1M table. Other standards list slightly different counterbore sizes, so use the one the drawing names.
What is the tap drill for the matching thread?
A #7 drill (0.201 in) for 1/4-20 and a 5.0 mm drill for M6 x 1, from the tap drill chart.
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