Westfield Fasteners Product Specification:

ASME B18.2.1 / ASME B18.6.3 - UNC Hex Head Set Screw

This product guide contains the specification for UNC threaded hex set screws, a series of standard parts available from Westfield Fasteners. The basis of this specification are the ASME standards ASME B18.2.1 and ASME B18.6.3.

Product Description

UNC hex head set screws are a fully threaded version of a hex bolt and are amongst the most common threaded fastener types. These hex head set screws are designed to be tightened with a standard spanner or ratchet and socket. These standard hex set screws are used in conjunction with UNC threaded nuts, and are used in a vast array of assemblies across the world. The external hexagonal drive allows for a greater torque load to be applied to the joint over most other drive types.

Scope of the ASME Standards

Unlike ISO standards, ASME standards usually cover a range or family of product types. ASME B18.2.1 covers many bolt types, including these UNC threaded hex set screws. The ASME B18.2.1 standard describes the dimensions and tolerances of hex head set screws with diameters beginning at 1/4 inch up to and including 4 inches. Hex head set screws with diameter sizes of 12 gauge and below are specified in ASME B18.6.3.

Tables 1 and 2 below define the dimensions and tolerances of these hex head screws to their respective standards.

technical drawing of UNC hex set screws, to ASME standard ASME B18.2.1 and ASME B18.6.3.

Figure 1: UNC Hex Head Set Screw

Material and Strength Specifications

ASME B18.6.3 - Diameters under 1/4"

Plain and zinc plated hex set screws are typically a non heat treated low carbon steel.

Stainless steel variants are typically produced to ASTM F837.

ASME B18.2.1 - Diameters 1/4" to 1-1/2"

Grade 5 plain and zinc plated carbon steel variants of these hex set screws are typically produced to SAE J429. This means that the minimum tensile strength is 120 ksi for diameters 1/4 to 1 inch and a minimum of 105 ksi for 1 inch to 1-1/2 inch.

Stainless steel items are typically produced to ASTM F593, with the tensile strength beginning at 65 ksi, depending on condition.

For brass, the manufacturer decides on the chemical and mechanical properties.

Table 1: Dimensions & Tolerances according to ASME B18.6.3 (inch)

ASME B18.6.3 - UNC Hex Machine Screws
Nominal Diameter	tpi	Body Diameter, E	Width Across Flats, A/F	Across Corners, W/G	Head Height, H
max	min	Basic	max	min	min	max	Basic	max	min
1	0.0730	64	-	-	-	0.125	0.120	0.134	-	-	0.044	0.036
2	0.0860	56	-	-	-	0.125	0.120	0.134	-	-	0.050	0.040
3	0.0990	48	-	-	-	0.188	0.181	0.202	-	-	0.055	0.044
4	0.1120	40	-	-	-	0.188	0.81	0.202	-	-	0.060	0.049
5	0.1250	40	-	-	-	0.188	0.181	0.202	-	-	0.070	0.058
6	0.1380	32	-	-	-	0.250	0.244	0.272	-	-	0.093	0.080
8	0.1640	32	-	-	-	0.250	0.244	0.272	-	-	0.110	0.096
10	0.1900	24	-	-	-	0.312	0.305	0.340	-	-	0.120	0.102
12	0.2160	24	-	-	-	0.312	0.305	0.340	-	-	0.155	0.139

ASME B18.6.3 - UNC Hex Machine Screws

Nominal Diameter

tpi

Body Diameter, E

Width Across Flats, A/F

Across Corners, W/G

Head Height, H

max

min

Basic

max

min

max

Basic

max

min

0.0730

0.125

0.120

0.134

0.044

0.036

0.0860

0.125

0.120

0.134

0.050

0.040

0.0990

0.188

0.181

0.202

0.055

0.044

0.1120

0.188

0.81

0.202

0.060

0.049

0.1250

0.188

0.181

0.202

0.070

0.058

0.1380

0.250

0.244

0.272

0.093

0.080

0.1640

0.250

0.244

0.272

0.110

0.096

0.1900

0.312

0.305

0.340

0.120

0.102

0.2160

0.312

0.305

0.340

0.155

0.139

Table 2: Dimensions & tolerances according to ASME B18.2.1 (inch)

ASME B18.2.1 - UNC Hex Set Screws
Nominal Size or Basic Product Diamter	tpi	Full-Size body Diameter, d	Width Across Flats, s	Width Across Corners, e	Head Height, k	Radius of Fillet, r	Nominal Thread Length for Bolt Lengths, Lt
Max	Min	Basic	Max	Min	Max	Min	Basic	Max	Min	Max	Min	6 in. and Shorter	Over 6 in.
_1/4 (0.2500)	20	0.260	0.237	_7/16	0.438	0.425	0.505	0.484	_11/64	0.188	0.150	0.03	0.01	0.750	1.000
_5/16 (0.3125)	18	0.324	0.298	_1/2	0.500	0.484	0.577	0.552	_7/32	0.235	0.195	0.03	0.01	0.875	1.125
_3/8 (0.3750)	16	0.388	0.360	_9/16	0.562	0.544	0.650	0.6200	_1/4	0.268	0.226	0.03	0.01	1.000	1.250
_7/16 (0.4375)	14	0.452	0.421	_5/8	0.625	0.603	0.722	0.687	_19/64	0.316	0.272	0.03	0.01	1.125	1.375
_1/2 (0.5000)	13	0.515	0.482	_3/4	0.750	0.725	0.866	0.826	_11/32	0.364	0.302	0.03	0.01	1.250	1.500
_5/8 (0.6250)	11	0.642	0.605	_5/16	0.938	0.906	1.083	1.033	_27/64	0.444	0.378	0.06	0.02	1.500	1.750
_3/4 (0.7500)	10	0.768	0.729	1_1/8	1.125	1.088	1.299	1.240	_1/2	0.524	0.455	0.06	0.02	1.750	2.000
_7/8 (0.8750)	9	0.895	0.895	1_5/16	1.312	1.269	1.516	1.447	_37/64	0.604	0.531	0.06	0.02	2.000	2.250
1 (1.0000)	8	1.022	1.022	1_1/2	1.500	1.450	1.732	1.653	_43/64	0.700	0.591	0.09	0.03	2.250	2.500
1_1/8 (1.1250)	7	1.149	1.149	1_11/16	1.688	1.631	1.949	1.859	_3/4	0.780	0.658	0.09	0.03	2.500	2.750
1_1/4 (1.2500)	7	1.277	1.277	1_7/8	1.875	1.812	2.165	2.066	_27/32	0.876	0.749	0.09	0.03	2.750	3.000
1_3/8 (1.3750)	6	1.404	1.404	2_1/16	2.062	1.994	2.382	2.273	_29/32	0.940	0.810	0.09	0.03	3.000	3.250
1_1/2 (1.5000)	6	1.531	1.531	2_1/4	2.250	2.175	2.598	2.480	1	1.036	0.902	0.09	0.03	3.250	3.500
1_5/8 (1.6250)	5-1/2	1.658	1.658	2_7/16	2.438	2.356	2.815	2.616	1_3/32	1.116	0.978	0.09	0.03	3.500	3.750
1_3/4 (1.7500)	5	1.785	1.785	2_5/8	2.625	2.538	3.031	2.893	1_5/32	1.196	1.054	0.12	0.04	3.750	4.000
1_7/8 (1.8750)	4-1/2	1.912	1.912	2_13/16	2.812	2.719	3.248	3.099	1_1/4	1.276	1.130	0.12	0.04	4.000	4.250

ASME B18.2.1 - UNC Hex Set Screws

Nominal Size or Basic Product Diamter

tpi

Full-Size body Diameter, d

Width Across Flats, s

Width Across Corners, e

Head Height, k

Radius of Fillet, r

Nominal Thread Length for Bolt Lengths, Lt

Max

Min

Basic

Max

Min

Max

Min

Basic

Max

Min

Max

Min

6 in. and Shorter

Over 6 in.

_1/4 (0.2500)

0.260

0.237

_7/16

0.438

0.425

0.505

0.484

_11/64

0.188

0.150

0.03

0.01

0.750

1.000

_5/16 (0.3125)

0.324

0.298

_1/2

0.500

0.484

0.577

0.552

_7/32

0.235

0.195

0.03

0.01

0.875

1.125

_3/8 (0.3750)

0.388

0.360

_9/16

0.562

0.544

0.650

0.6200

_1/4

0.268

0.226

0.03

0.01

1.000

1.250

_7/16 (0.4375)

0.452

0.421

_5/8

0.625

0.603

0.722

0.687

_19/64

0.316

0.272

0.03

0.01

1.125

1.375

_1/2 (0.5000)

0.515

0.482

_3/4

0.750

0.725

0.866

0.826

_11/32

0.364

0.302

0.03

0.01

1.250

1.500

_5/8 (0.6250)

0.642

0.605

_5/16

0.938

0.906

1.083

1.033

_27/64

0.444

0.378

0.06

0.02

1.500

1.750

_3/4 (0.7500)

0.768

0.729

1_1/8

1.125

1.088

1.299

1.240

_1/2

0.524

0.455

0.06

0.02

1.750

2.000

_7/8 (0.8750)

0.895

1_5/16

1.312

1.269

1.516

1.447

_37/64

0.604

0.531

0.06

0.02

2.000

2.250

1 (1.0000)

1.022

1_1/2

1.500

1.450

1.732

1.653

_43/64

0.700

0.591

0.09

0.03

2.250

2.500

1_1/8 (1.1250)

1.149

1_11/16

1.688

1.631

1.949

1.859

_3/4

0.780

0.658

0.09

0.03

2.500

2.750

1_1/4 (1.2500)

1.277

1_7/8

1.875

1.812

2.165

2.066

_27/32

0.876

0.749

0.09

0.03

2.750

3.000

1_3/8 (1.3750)

1.404

2_1/16

2.062

1.994

2.382

2.273

_29/32

0.940

0.810

0.09

0.03

3.000

3.250

1_1/2 (1.5000)

1.531

2_1/4

2.250

2.175

2.598

2.480

1.036

0.902

0.09

0.03

3.250

3.500

1_5/8 (1.6250)

5-1/2

1.658

2_7/16

2.438

2.356

2.815

2.616

1_3/32

1.116

0.978

0.09

0.03

3.500

3.750

1_3/4 (1.7500)

1.785

2_5/8

2.625

2.538

3.031

2.893

1_5/32

1.196

1.054

0.12

0.04

3.750

4.000

1_7/8 (1.8750)

4-1/2

1.912

2_13/16

2.812

2.719

3.248

3.099

1_1/4

1.276

1.130

0.12

0.04

4.000

4.250

Table 2: Dimensions & tolerances according to ASME B18.2.1 (inch) Continued

ASME B18.2.1 - UNC Hex Set Screws
Nominal Size or Basic Product Diamter	tpi	Full-Size body Diameter, d	Width Across Flats, s	Width Across Corners, e	Head Height, k	Radius of Fillet, r	Nominal Thread Length for Bolt Lengths, Lt
Max	Min	Basic	Max	Min	Max	Min	Basic	Max	Min	Max	Min	6 in. and Shorter	Over 6 in.
2 (2.0000)	4-1/2	2.039	2.039	3	3.000	2.900	3.464	3.306	1_11/32	1.388	1.175	0.12	0.04	4.250	4.500
2_1/4 (2.2500)	4-1/2	2.305	2.305	3_3/8	3.375	3.262	3.897	3.719	1_1/2	1.548	1.327	0.19	0.06	4.750	5.000
2_1/2 (2.5000)	4	2.559	2.559	3_3/4	3.750	3.625	4.330	4.133	1_21/32	1.708	1.479	0.19	0.06	5.250	5.500
2_3/4 (2.7500)	4	2.827	2.827	4_1/8	4.125	3.988	4.763	4.546	1_13/16	1.869	1.632	0.19	0.06	5.750	6.000
3 (3.0000)	4	3.081	3.081	4_1/2	4.500	4.350	5.196	4.959	2	2.060	1.815	0.19	0.06	6.250	6.500
3_1/4 (3.2500)	4	3.335	3.335	4_7/8	4.875	4.712	5.629	5.372	2_3/16	2.251	1.936	0.19	0.06	6.750	7.000
3_1/2 (3.5000)	4	3.589	3.589	5_1/4	5.250	5.075	6.062	5.786	2_5/16	2.380	2.057	0.19	0.06	7.250	7.500
3_3/4 (3.7500)	4	3.858	3.858	5_5/8	5.625	5.437	6.495	6.198	2_1/2	2.572	2.241	0.19	0.06	7.750	8.000
4 (4.0000)	4	4.111	4.111	6	6.000	5.800	6.928	6.612	2_11/16	2.764	2.424	0.19	0.06	8.250	8.500

ASME B18.2.1 - UNC Hex Set Screws

Nominal Size or Basic Product Diamter

tpi

Full-Size body Diameter, d

Width Across Flats, s

Width Across Corners, e

Head Height, k

Radius of Fillet, r

Nominal Thread Length for Bolt Lengths, Lt

Max

Min

Basic

Max

Min

Max

Min

Basic

Max

Min

Max

Min

6 in. and Shorter

Over 6 in.

2 (2.0000)

4-1/2

2.039

3.000

2.900

3.464

3.306

1_11/32

1.388

1.175

0.12

0.04

4.250

4.500

2_1/4 (2.2500)

4-1/2

2.305

3_3/8

3.375

3.262

3.897

3.719

1_1/2

1.548

1.327

0.19

0.06

4.750

5.000

2_1/2 (2.5000)

2.559

3_3/4

3.750

3.625

4.330

4.133

1_21/32

1.708

1.479

0.19

0.06

5.250

5.500

2_3/4 (2.7500)

2.827

4_1/8

4.125

3.988

4.763

4.546

1_13/16

1.869

1.632

0.19

0.06

5.750

6.000

3 (3.0000)

3.081

4_1/2

4.500

4.350

5.196

4.959

2.060

1.815

0.19

0.06

6.250

6.500

3_1/4 (3.2500)

3.335

4_7/8

4.875

4.712

5.629

5.372

2_3/16

2.251

1.936

0.19

0.06

6.750

7.000

3_1/2 (3.5000)

3.589

5_1/4

5.250

5.075

6.062

5.786

2_5/16

2.380

2.057

0.19

0.06

7.250

7.500

3_3/4 (3.7500)

3.858

5_5/8

5.625

5.437

6.495

6.198

2_1/2

2.572

2.241

0.19

0.06

7.750

8.000

4 (4.0000)

4.111

6.000

5.800

6.928

6.612

2_11/16

2.764

2.424

0.19

0.06

8.250

8.500