Socket head cap screw size chart

Head diameters and heights, hex key sizes for cap, button, flat and set screws, and counterbore drills.

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Unified inch · ASME B1.1

1/4"-20UNC

0.250" / 6.35 mm major diameter

Tap drill · 75%

0.201" · 5.11 mm

75.4% thread

Hardware-store bit

13/64"

0.2031"

72.2% thread

Wrench / hex key

7/16"nut3/16"Allen

Drills & threading

Major diameter: 0.250"6.35 mm · nearest 1/4"
Pitch: 20 TPI UNC0.0500" · 1.27 mm
Tap drill 75%: #7 · 75.4%0.2010" · 5.11 mm
Hardware-store bit: 13/64" · 72.2%0.2031" from a 1/64" set
Form / roll tap 65%: #10.2280" · 5.79 mm
STI / Helicoil: H1/4"-20 STI tap · 0.2660"
Clearance · close / free: F / H0.2570" / 0.2660"
Counterbore: 7/16"0.438" · 11.11 mm

Wrenches, keys & torque

Hex head: 7/16"head 0.156" · 3.97 mm tall
Nut: 7/16"0.219" · 5.56 mm thick · flange OD 0.590" · 14.99 mm
Socket cap key: 3/16"head ⌀ 0.375" · 9.52 mm
Button / flat · set: 5/32" · 1/8"
Flat washer SAE: 0.281" / 0.625"USS: 0.312" / 0.734"
SAE Gr 2 · Gr 5: 66 in-lb · 101 in-lb
SAE Grade 8: 12 ft-lb
18-8 stainless: 78 in-lbGalling risk: use anti-seize on stainless and tighten slowly.

Full chart 56 rows

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Socket head cap, button, flat & set screw chart

56 rows

Socket head cap, button, flat & set screw chart · ScrewBoltChart

screwboltchart.com · inch-first · tap drills 75% (soft metals) · torque dry, K=0.20 · verify against the governing standard


0.096"	0.060"	0.050"	0.035"	0.028"	1/8"
0.118"	0.073"	1/16"	0.050"	0.035"	5/32"
0.118"	0.073"	1/16"	0.050"	0.035"	5/32"
0.140"	0.086"	5/64"	0.050"	0.035"	3/16"
0.140"	0.086"	5/64"	0.050"	0.035"	3/16"
0.161"	0.099"	5/64"	1/16"	0.050"	7/32"
0.161"	0.099"	5/64"	1/16"	0.050"	7/32"
0.183"	0.112"	3/32"	1/16"	0.050"	7/32"
0.183"	0.112"	3/32"	1/16"	0.050"	7/32"
0.205"	0.125"	3/32"	5/64"	1/16"	1/4"
0.205"	0.125"	3/32"	5/64"	1/16"	1/4"
0.226"	0.138"	7/64"	5/64"	1/16"	9/32"
0.226"	0.138"	7/64"	5/64"	1/16"	9/32"
0.270"	0.164"	9/64"	3/32"	5/64"	5/16"
0.270"	0.164"	9/64"	3/32"	5/64"	5/16"
0.312"	0.190"	5/32"	1/8"	3/32"	3/8"
0.312"	0.190"	5/32"	1/8"	3/32"	3/8"
N/A	N/A	N/A	N/A	N/A	N/A
N/A	N/A	N/A	N/A	N/A	N/A
0.375"	0.250"	3/16"	5/32"	1/8"	7/16"
0.375"	0.250"	3/16"	5/32"	1/8"	7/16"
0.469"	0.313"	1/4"	3/16"	5/32"	17/32"
0.469"	0.313"	1/4"	3/16"	5/32"	17/32"
0.563"	0.375"	5/16"	7/32"	3/16"	5/8"
0.563"	0.375"	5/16"	7/32"	3/16"	5/8"
0.656"	0.438"	3/8"	1/4"	7/32"	23/32"
0.656"	0.438"	3/8"	1/4"	7/32"	23/32"
0.750"	0.500"	3/8"	5/16"	1/4"	13/16"
0.750"	0.500"	3/8"	5/16"	1/4"	13/16"
N/A	N/A	N/A	N/A	N/A	N/A
N/A	N/A	N/A	N/A	N/A	N/A
0.938"	0.625"	1/2"	3/8"	5/16"	1"
0.938"	0.625"	1/2"	3/8"	5/16"	1"
1.125"	0.750"	5/8"	1/2"	3/8"	1-3/16"
1.125"	0.750"	5/8"	1/2"	3/8"	1-3/16"
1.312"	0.875"	3/4"	9/16"	1/2"	1-3/8"
1.312"	0.875"	3/4"	9/16"	1/2"	1-3/8"
1.500"	1.000"	3/4"	5/8"	9/16"	1-5/8"
1.500"	1.000"	3/4"	5/8"	9/16"	1-5/8"
0.118"	0.063"	1.5 mm	N/A	0.7 mm	—
0.150"	0.079"	1.5 mm	N/A	0.9 mm	—
0.177"	0.098"	2 mm	N/A	1.3 mm	—
0.217"	0.118"	2.5 mm	2 mm	1.5 mm	—
0.236"	0.138"	2.5 mm	N/A	1.5 mm	—
0.276"	0.157"	3 mm	2.5 mm	2 mm	—
0.335"	0.197"	4 mm	3 mm	2.5 mm	—
0.394"	0.236"	5 mm	4 mm	3 mm	—
0.512"	0.315"	6 mm	5 mm	4 mm	—
0.630"	0.394"	8 mm	6 mm	5 mm	—
0.709"	0.472"	10 mm	8 mm	6 mm	—
0.827"	0.551"	12 mm	10 mm	6 mm	—
0.945"	0.630"	14 mm	10 mm	8 mm	—
1.063"	0.709"	14 mm	12 mm	8 mm	—
1.181"	0.787"	17 mm	12 mm	10 mm	—
1.299"	0.866"	17 mm	14 mm	10 mm	—
1.417"	0.945"	19 mm	14 mm	12 mm	—

05 / Field guide

The right key,
first try.

Socket head cap screws are the precision fastener of machine building: a strong alloy-steel screw with a cylindrical head and a hex socket. Button, flat and set screws share the same keys in a different pattern.

Proportions you can count on

An inch cap screw head is about 1.5 × the diameter across and exactly 1 × the diameter tall. A 1/4" cap screw has a 0.375" head, 0.25" tall, and takes a 3/16" key. Metric ISO 4762 follows the same idea: an M6 has a 10 mm head, 6 mm tall, and takes a 5 mm key.

Button, flat and set screws use smaller keys

Lower heads have shallower sockets, so they use a smaller key than the cap screw of the same size. A 1/4" button or flat head takes 5/32"; a 1/4" set screw takes 1/8". In metric, an M6 button head takes 4 mm and an M6 set screw 3 mm.

Counterbores

To sink a cap screw flush, drill a counterbore slightly larger than the head and as deep as the head height. For 1/4" use a 7/16" counterbore; for #10, 3/8".

Strength and tightening

Inch socket head cap screws are alloy steel to ASTM A574, stronger than a Grade 8 hex bolt; metric ones are usually class 12.9. That strength is why their heads can be so compact. Torque them with a proper hex bit in a torque wrench rather than a bent L-key: the short arm of an L-key gives too little feel, and the long arm can overload small screws. Replace keys as soon as the corners round.

Questions

Answered plainly.

01What Allen key fits a 1/4-20 socket head cap screw?+

A 3/16" hex key. A 1/4" button or flat head takes 5/32", and a 1/4" set screw takes 1/8".

02What Allen key fits an M5 cap screw?+

A 4 mm hex key for ISO 4762 socket head cap screws. M5 button and flat heads take 3 mm.

03What Allen key fits a #10 cap screw?+

A 5/32" key. #10 button and flat heads take 1/8", and #10 set screws take 3/32".

04Can I use a metric key in an inch screw?+

Only where sizes nearly match. 5 mm (0.197") is too large for 3/16" (0.1875"); 4.5 mm doesn’t exist in most sets. Use the right system, or the corners round off.

05Why is my Allen key slipping?+

Usually a worn key, a mixed-system key, or debris in the socket. Clean the socket, use a key with sharp corners and seat it fully before applying torque.

06How do I size a counterbore?+

Diameter slightly over the head (for a 1/4" cap screw, 7/16"), depth equal to the head height plus any clearance you want below the surface.

07What Allen key fits a 3/8-16 cap screw?+

A 5/16" hex key. 3/8" button and flat heads take 7/32", and 3/8" set screws take 3/16".

08What Allen key fits an M8 cap screw?+

A 6 mm key. M8 button and flat heads take 5 mm, and M8 set screws take 4 mm.

09How tall is a socket head cap screw head?+

The same as its diameter: a 1/4" cap screw has a 0.25" tall head and an M6 a 6 mm head. That is the depth to counterbore for a flush fit.

10What is a low-head cap screw?+

A socket cap screw with a head about two-thirds the normal height, for tight spaces. It takes a smaller key and carries less torque than a standard cap screw of the same size.

11What counterbore for an M6 cap screw?+

An M6 ISO 4762 head is 10 mm across and 6 mm tall, so an 11 mm counterbore 6.5 mm deep sits it just below the surface.