Nuts · ANSI/ASME B18.2.2 / BS 1768

BS 1768 Imperial Hex Nuts (UNC / UNF) Specification

Dimensional table, tolerance classes and the DIN / ISO / ASME parts that supersede or match this standard.

BS 1768 specifies the dimensions and tolerances for imperial hex nuts (unc / unf). The full dimensional table is reproduced below, followed by the DIN, ISO and ASME parts that match or supersede it.

Standard: BS 1768
Product: Imperial Hex Nuts (UNC / UNF)
Governing standards: ANSI/ASME B18.2.2 / BS 1768

Equivalence between national standards is dimensional, not always metallurgical. Check the property class separately when substituting.

BS 1768 is the governing dimensional standard for imperial hex nuts (unc / unf). This page sets out the scope of the standard, reproduces the full size table it specifies, and lists the DIN/ISO/ASME cross-references commonly cited in datasheets and drawings.

BS 1768 — Scope

BS 1768 specifies the dimensions and tolerances of imperial hex nuts (unc / unf). Material, property class, surface finish and packaging are governed by separate documents in the same standards family (for example ISO 898-1 for the mechanical property classes of bolts and screws).

Tolerance & Product Grades

BS 1768, alongside the related ANSI/ASME B18.2.2 / BS 1768 entries on this site, specifies product grades (typically A or B for fine work and C for general engineering) and tolerance classes for the controlling dimensions. Choose the grade against the precision your application requires.

Dimensions Specified by BS 1768

Nominal size	Thread series	Material options	Finish options
1/4 in	UNC or UNF	carbon steel or A2/A4 stainless	black oxide / blue-white zinc / yellow zinc / hot-dip galvanized
5/16 in	UNC or UNF	carbon steel or A2/A4 stainless	black oxide / blue-white zinc / yellow zinc / hot-dip galvanized
3/8 in	UNC or UNF	carbon steel or A2/A4 stainless	black oxide / blue-white zinc / yellow zinc / hot-dip galvanized
1/2 in	UNC or UNF	carbon steel or A2/A4 stainless	black oxide / blue-white zinc / yellow zinc / hot-dip galvanized
5/8 in	UNC or UNF	carbon steel or A2/A4 stainless	black oxide / blue-white zinc / hot-dip galvanized
3/4 in	UNC or UNF	carbon steel or A2/A4 stainless	black oxide / blue-white zinc / hot-dip galvanized

Companion Standards

BS 1768 controls only the geometry of imperial hex nuts (unc / unf). The mechanical strength is set by ISO 898-1 (bolts and screws) or ISO 898-2 (nuts); the surface finish is governed by separate finish standards (e.g. ISO 4042 for electroplating). For corrosion testing, the relevant document is ISO 9227 (neutral salt spray).

When to Specify BS 1768

Choose BS 1768 for international engineering work and OEM supply where ISO standards are the contractual reference. It is the default modern specification for imperial hex nuts (unc / unf).

Technical Drawing

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Specification

Item / category Standard Size(s) Material / finish Quantity

Business Information

Company * Contact Name * Email * Phone (with country code) Country / Region

Notes

Dimensions & Specifications (ANSI/ASME B18.2.2 / BS 1768)

Nominal size	Thread series	Material options	Finish options
1/4 in	UNC or UNF	carbon steel or A2/A4 stainless	black oxide / blue-white zinc / yellow zinc / hot-dip galvanized
5/16 in	UNC or UNF	carbon steel or A2/A4 stainless	black oxide / blue-white zinc / yellow zinc / hot-dip galvanized
3/8 in	UNC or UNF	carbon steel or A2/A4 stainless	black oxide / blue-white zinc / yellow zinc / hot-dip galvanized
1/2 in	UNC or UNF	carbon steel or A2/A4 stainless	black oxide / blue-white zinc / yellow zinc / hot-dip galvanized
5/8 in	UNC or UNF	carbon steel or A2/A4 stainless	black oxide / blue-white zinc / hot-dip galvanized
3/4 in	UNC or UNF	carbon steel or A2/A4 stainless	black oxide / blue-white zinc / hot-dip galvanized

Materials & Grades

Select a material class to see the grades we supply it in. Each designation is listed with its ISO/EN, AISI/SAE, JIS and GB equivalents.

Stainless Steel38 Carbon Steel66 Alloy Steel41 Bearing Steel5 Spring Steel25 Aluminum and Aluminum Alloys21 Brass17 Bronze11 Copper8 Titanium and Titanium Alloys18 Nickel and Nickel Alloys22 Iron-Nickel and High-Temperature Alloys10 Tool Steel and Specialty Hardened Steel9 Structural and Construction Steel9

Stainless Steel

Austenitic Stainless Steel

A1 AISI 303 · SAE 303 · SUS303 · EN 1.4305 · X8CrNiS18-9
A2 AISI 304 · SAE 304 · SUS304 · EN 1.4301 · X5CrNi18-10 · 06Cr19Ni10
304L AISI 304L · SUS304L · EN 1.4307 · X2CrNi18-9 · 022Cr19Ni10
302 AISI 302 · SUS302 · EN 1.4310
305 AISI 305 · SUS305 · EN 1.4303
A4 AISI 316 · SAE 316 · SUS316 · EN 1.4401 · X5CrNiMo17-12-2 · 06Cr17Ni12Mo2
316L AISI 316L · SUS316L · EN 1.4404 · X2CrNiMo17-12-2 · 022Cr17Ni12Mo2
321 AISI 321 · SUS321 · EN 1.4541 · X6CrNiTi18-10
310 / 310S AISI 310 · AISI 310S · SUS310S · EN 1.4845
302HQ AISI 302HQ
A3
A5
347 AISI 347 · SUS347 · EN 1.4550 · X6CrNiNb18-10
309 / 309S AISI 309 · AISI 309S · SUS309S

Martensitic Stainless Steel

AISI 410 SUS410 · EN 1.4006 · X12Cr13 · 12Cr13
AISI 420 SUS420J1 · SUS420J2 · EN 1.4021 · EN 1.4028 · 20Cr13 · 30Cr13
AISI 431 SUS431 · EN 1.4057 · X17CrNi16-2
C1
C3
C4
AISI 440A
AISI 440B
AISI 440C SUS440C · EN 1.4125

Ferritic Stainless Steel

AISI 430 SUS430 · EN 1.4016 · X6Cr17 · 10Cr17
F1
AISI 434
AISI 444

Precipitation-Hardening Stainless Steel

17-4 PH AISI 630 · UNS S17400 · SUS630 · EN 1.4542 · X5CrNiCuNb16-4
15-5 PH UNS S15500
17-7 PH UNS S17700

Duplex and Super Duplex Stainless Steel

Duplex 2205 UNS S31803 · UNS S32205 · EN 1.4462
Super Duplex 2507 UNS S32750 · EN 1.4410
D2
D4
D6
D8
Lean Duplex 2101 UNS S32101
Super Duplex S32760 UNS S32760 · EN 1.4501

Carbon Steel

Low-Carbon Steel

SAE 1006
SAE 1008
SAE 1010
SAE 1018
SAE 1020
Q195
Q235
SAE 1012
SAE 1015
Q215
ML08
ML08Al
ML10
ML15
ML20
SWRCH6A
SWRCH8A
SWRCH10A
SWRCH12A
SWRCH15A
SWRCH18A

Medium-Carbon Steel

SAE 1035
SAE 1045
S45C
C45
SAE 1030
SAE 1040
SAE 1050
30
35
40
45
50
S35C
S40C
S50C
C35
C40

High-Carbon Steel

SAE 1060
SAE 1070
65Mn
SAE 1065
SAE 1075
SAE 1080
SAE 1085
SAE 1090
SAE 1095
60
65
70
75
80
85

Cold-Heading Steel

SWRCH22A
10B21
SWRCH22K
SWRCH25K
SWRCH30K
SWRCH35K
SWRCH40K
ML20Mn
ML25
ML30
ML35
ML40
15B25

Alloy Steel

Chromium Steel

40Cr
45Cr
AISI 5140
SAE 5140
AISI 5120
SAE 5120
SCr420
SCr440

Chromium-Molybdenum Steel

35CrMo
42CrMo
AISI 4140
SCM435
SCM440
42CrMo4
30CrMo
AISI 4130
SAE 4130
SAE 4140
AISI 4142
AISI 4145
SCM415
SCM420
SCM445
EN 1.7220
EN 1.7225
25CrMo4
34CrMo4

Nickel-Chromium-Molybdenum Steel

AISI 4340
SAE 4340
40CrNiMo
SNCM439
EN 1.6511
36CrNiMo4

Boron Alloy Steel

10B22
10B21
15B21
15B25
20MnTiB
20Mn2B
35VB
40MnB

Bearing Steel

GCr15
AISI 52100 SAE 52100 · UNS G52986
SUJ2
100Cr6 EN 1.3505
GCr15SiMn

Spring Steel

Carbon Spring Steel

SAE 1065
SAE 1070
SAE 1075
SAE 1080
SAE 1095
65
70
75
80
65Mn

Silicon-Manganese Spring Steel

60Si2Mn
60Si2MnA
SUP6
SUP7

Chromium-Vanadium Spring Steel

50CrVA
SAE 6150
AISI 6150
SUP10

Stainless Spring Steel

AISI 301
SUS301
EN 1.4310
AISI 302
AISI 304
AISI 316
17-7 PH

Aluminum and Aluminum Alloys

Commercially Pure Aluminum

1050
1060
1070
1100

2xxx Series Aluminum

2024
2011
2014
2017

5xxx Series Aluminum

5052
5083
5754

6xxx Series Aluminum

6061
6061-T6
6063
6082
6082-T6

7xxx Series Aluminum

7075
7075-T6
7005
7050
7075-T73

Brass

Free-Cutting Brass

C36000
H62
HPb59-1
C3601
C3602
C3604
CuZn36Pb3
CuZn39Pb3
EN CW614N
EN CW617N
H65

Lead-Free Brass

C69300
C27450
CW510L
CW511L

Naval Brass

C46400
CuZn39Sn1

Bronze

Silicon Bronze

C65500
C65100
C65600

Phosphor Bronze

C51000
C52100
CuSn6
CuSn8

Aluminum Bronze

C95400
C95500
C63000
CuAl10Ni5Fe4

Copper

C11000
T2
C10100
C10200
C12200
T1
TU1
TU2

Titanium and Titanium Alloys

Commercially Pure Titanium

Grade 2
Grade 1
Grade 3
Grade 4
TA1
TA2
TA3

Alpha-Beta Titanium Alloys

Grade 5
Ti-6Al-4V
TC4
UNS R56400

Low-Interstitial Grade

Grade 23
Ti-6Al-4V ELI
UNS R56407

Other Titanium Fastener Alloys

Ti-6Al-6V-2Sn
Ti-6Al-2Sn-4Zr-2Mo
Beta-C titanium
Ti-10V-2Fe-3Al

Nickel and Nickel Alloys

Commercially Pure Nickel

Nickel 200
UNS N02200
Nickel 201
UNS N02201

Inconel Alloys

Inconel 625 UNS N06625
Inconel 718 UNS N07718
Inconel 600 UNS N06600
Inconel 601 UNS N06601
Inconel 686 UNS N06686
Inconel X-750 UNS N07750

Monel Alloys

Monel 400 UNS N04400
Monel K-500 UNS N05500

Hastelloy Alloys

Hastelloy C-276 UNS N10276
Hastelloy C-22 UNS N06022
Hastelloy B-2 UNS N10665
Hastelloy B-3 UNS N10675
Hastelloy X UNS N06002

Other Nickel Alloys

Alloy 20 UNS N08020
Alloy 625
Alloy 718
Alloy 825 UNS N08825
Alloy 925 UNS N09925

Iron-Nickel and High-Temperature Alloys

A286

A-286
UNS S66286
EN 1.4980

Invar

Invar 36
Alloy 36
UNS K93600
FeNi36

Alloy 42

Alloy 42
UNS K94100
FeNi42

Tool Steel and Specialty Hardened Steel

AISI D2
AISI H13
SKD11
Cr12MoV
AISI O1
AISI A2
AISI S7
SKD61
9CrSi

Structural and Construction Steel

ASTM A36
Q235B
Q355B
S355JR
Q235C
Q355C
S235JR
S275JR
S355J2

Applications & Industries

The sectors that buy Imperial Hex Nuts to ANSI/ASME B18.2.2 / BS 1768, and the duty each one buys them for. Stocked from 1/4 in to 3/4 in (screw size).

Industries that specify Imperial Hex Nuts

Marine hardware A4 stainless for salt exposure on inch-threaded fittings.
Legacy equipment Keeps older plant serviceable without re-threading.
Overseas projects UNC and UNF supplied against the project standard.
Imported machinery Inch threads do not mate with metric hardware of similar size.
Machine building & OEM The clamping counter-face on studs and bolts throughout an assembly.
Steel fabrication Bolted structural and frame connections.

Typical applications

selecting the finish for the service environment
keeping inch and metric stock physically separated
checking across-flats against the available spanner
matching UNC or UNF to the existing bolt, not to the nominal size alone
completing a bolted joint on a stud, rod or bolt
locking an adjustment so it cannot drift

Service conditions to specify against

Marine and coastal: Available in A4 (316) stainless for chloride exposure — coastal structures, pool halls, water treatment and deck hardware, where A2 will eventually pit.
High load: Supplied in ISO 898-1 property classes, so the joint can be designed to a stated proof load and tightened to a calculated torque rather than by feel.
Exterior structural: Hot-dip galvanised options for external steelwork; allow for the thicker coating in the mating thread.
Match the class: A nut below the bolt property class strips before the bolt reaches its own limit — pair class 8 nuts with 8.8 bolts, class 10 with 10.9.

Where to use something else

Do not run a metric nut onto an inch bolt because it starts; it will strip the thread and the joint will fail.

Full application guide: where Imperial Hex Nuts are used, how to specify them, and the cases they are wrong for

Inside the plant

Manufactured in our own plant

This part is cold-formed, threaded, heat-treated and finished in-house. The stages below are where its class and finish are actually decided.

01

Cold heading

Wire rod is drawn to size and cold-formed into blanks. Forming rather than cutting keeps the grain flow continuous around the head, which is where a bolt is most likely to fail.

Head dimensions and under-head radius checked at set-up and per shift.
02

Thread rolling

Threads are rolled between hardened dies rather than cut. Rolled threads are stronger in fatigue than cut threads and hold pitch and flank angle far more consistently across a batch.

Go / no-go ring gauges to ISO 965 tolerance class 6g.
03

Heat treatment

Quench-and-temper brings carbon and alloy steel to the property class ordered — 8.8, 10.9 or 12.9 — with the furnace charge tracked so a finished lot can be traced back to it.

Core hardness and tensile tested per lot to ISO 898-1.
04

Surface treatment

Zinc, zinc-nickel, hot-dip galvanizing, black oxide, phosphate and Dacromet, selected against the service environment rather than against the price list.

Coating thickness and neutral salt spray hours to ISO 9227.

Frequently Asked Questions

What does BS 1768 specify for Imperial Hex Nuts (UNC / UNF)?

BS 1768 specifies the dimensions and tolerances of imperial hex nuts (unc / unf). Material and property class are governed by separate standards in the same family.

What product grades does BS 1768 define?

BS 1768 typically distinguishes product grades for precision (A/B) and general engineering (C). The selection for Imperial Hex Nuts (UNC / UNF) depends on the precision your application requires.

Does BS 1768 specify the mechanical strength?

No. BS 1768 controls only the geometry of imperial hex nuts (unc / unf). The mechanical strength is set by ISO 898-1 (bolts / screws) or ISO 898-2 (nuts), and the finish is governed by separate standards.

Can I substitute BS 1768 for the corresponding DIN standard on a drawing?

On modern drawings the ISO standard is the master reference; the older DIN equivalent is a cross-reference. For most dimensional purposes the parts are interchangeable, but check tolerance grades and the head height — these are the most common points of divergence.

How do I request a quotation?

Use the technical drawing on this page to pick the exact configuration, then submit it with the quote form. We respond within one business day.