How to Specify a Bolt: Size, Thread, Grade, and Finish

The short answer: specify the bolt’s product standard, complete thread size, length, strength grade and finish. Add the quantity, mating hardware and required inspection documents. Name a particular steel alloy when the drawing requires one, and include the drawing number and revision if they control the part.

A callout such as 1/2-13 UNC-2A × 3 in, SAE Grade 8 or M12 × 1.75 × 80, 10.9 identifies only part of the order. This guide works through Unified-inch and Chinese GB/T metric examples to show what to add.

Start with the question you need to answer

• Trying to decode an existing size? Read How to Read Bolt Sizes: Metric and Inch Callouts Explained for diameter, pitch, TPI, thread class, and length.
• Choosing between UNC and UNF? Read UNC and UNF Threads: How Pitch Changes Joint Performance for coarse- and fine-thread selection within the Unified-inch system.
• Not sure which grade system applies? Start with Fastener Grades Explained: What 8.8, Grade 8, A2-70 and B7 Mean, then choose the focused article for the marking on the drawing.
• Interpreting a metric property class? Read Bolt Grades 8.8, 10.9 and 12.9: What the Numbers Really Mean for the two-number strength code and what it does not specify.
• Comparing SAE inch grades? Read Grade 5 vs Grade 8 Bolts: Strength, Markings and Applications for SAE J429 strength, head markings, and common applications.
• Reading a stainless marking? Read A2-70 and A4-80: How to Read Stainless Fastener Markings for the stainless group and tensile property class.

For a complete purchase description, work through the sections below: product and dimensions, thread, strength and material, finish, mating parts, then inspection.

What makes a bolt specification complete?

1/2-13 identifies an inch thread’s diameter and threads per inch (TPI); M12 identifies a metric thread’s nominal diameter. Add the product form, length, grade, finish and inspection requirements to describe the part you need.

Use these eight fields to build the order:

Keep the product, thread, mechanical-property and coating requirements distinct. ASME B18.24 and GB/T 1237 provide fastener designation systems that link these requirements to their source standards.^{[1] [2]}

For the hex-head products here, measure nominal length from the bearing surface under the head to the end. If the joint needs a particular unthreaded grip or usable thread length, specify it separately; other head forms can use a different length datum.^{[3] [4]}

A supplier should be able to select the product form, thread fit, grip length and finish directly from the order. Resolve any open fields before comparing quotations.

What does a Unified-inch drawing require?

Read this partial description field by field:

Hex cap screw to ASME B18.2.1, 1/2-13 UNC-2A × 3 in, SAE J429 Grade 8, zinc-flake coated

The thread follows ASME B1.1-2024; the product dimensions follow ASME B18.2.1-2012 (R2021); Grade 5 and Grade 8 material and mechanical requirements follow SAE J429, revised May 2014.^{[3] [5] [6]}

Carry the complete thread designation onto the purchase order. A 1/2-13 UNC bolt and a 1/2-20 UNF bolt share a nominal diameter but need different mating threads. In a numbered size such as #10-24 UNC, #10 identifies diameter and 24 identifies TPI.

Start with the application specification when it controls the joint. ASTM A193/A193M-26 covers alloy- and stainless-steel bolting for high-temperature, high-pressure and other special-purpose service; B7 and B8 belong to that system.^[7]

For pressure, temperature, or other A193 service, start with the ASTM A193 and A194 Guide instead of substituting an SAE grade by number.

What changes on a Chinese GB/T metric drawing?

A metric thread can appear on products made to GB/T, ISO or DIN dimensions, so name the product standard as well as the size. GB/T 5782-2025 covers coarse-pitch hexagon-head bolts; GB/T 5785-2025 covers the fine-pitch products in its scope.^{[4] [8]}

For example, this description leaves the thread tolerance and full finish requirement to be added:

Hexagon head bolt to GB/T 5782-2025, M12 × 1.75 × 80, property class 10.9, zinc-flake coated

How do pitch and tolerance appear?

Metric pitch is the distance between adjacent threads, in millimeters. M12 × 1.75 and M12 × 1.25 need different nuts or tapped holes. Check the pitch allowed by the product standard against the diameter and pitch series in GB/T 193-2003.^[9]

Coarse pitch is implicit in many standard product designations. Write it out when replacing an unidentified part, transferring a drawing between standards systems or specifying a fine thread.

M12 × 1.75-6g includes an external-thread tolerance class; 6H is a common internal class. These control thread dimensions and fit. GB/T 197-2018 defines the Chinese general-purpose metric tolerance system.^[10]

Does the property class also specify the steel?

Specify the finished bolt’s property class with its governing standard: GB/T 3098.1-2010 for the Chinese route or ISO 898-1:2013 for the ISO route. Classes 8.8, 10.9 and 12.9 describe required mechanical properties.^{[11] [12]}

An alloy callout adds a different requirement. 35CrMo to GB/T 3077-2015 controls the steel; a bolt made from it must still meet the specified finished-fastener class after manufacture and heat treatment.^[13]

If the material designation is the uncertain part, How to Read Chinese Alloy Structural Steel Grades separates 35CrMo and 42CrMo chemistry from finished-fastener performance.

For stainless fasteners, A2-70 combines a stainless grade group with a tensile property class. A drawing requiring Type 304 chemistry needs that material requirement as well. GB/T 3098.6-2023 covers stainless bolt, screw and stud mechanical properties in the Chinese system.^[14]

If the open question is corrosion resistance, 304 vs 316 Stainless Steel Fasteners compares the two alloy families without treating them as property classes.

Can similar grade numbers be treated as equivalents?

Treat a cross-system substitution as an engineering change. First identify the kind of requirement each designation expresses:

Fastener Grades Explained provides the cross-system overview; the distinctions below show where each callout fits in a complete specification.

• Material: 35CrMo and 42CrMo are Chinese alloy-steel grades under GB/T 3077. Type 304 and Type 316 are shorthand alloy-family labels; they do not define a finished-fastener property class or a complete material procurement specification.
• Metric fastener performance: 8.8, 10.9, and 12.9 define properties for carbon- or alloy-steel fasteners. A2-70 and A4-80 combine stainless steel grades A2 and A4 with property classes 70 and 80.
• Unified fastener performance: SAE J429 Grade 5 and Grade 8 apply to products within J429's scope.
• Application-specific bolting: ASTM A193 Grade B8 identifies a Type 304 stainless bolting-material route for A193 service. State the applicable class or condition along with the grade.

Compare the actual standards and dimensions before approving an alternative. Metric class 10.9 and SAE Grade 8 have similar strength ranges, but different threads, product dimensions and markings. GB/T 5782-2025, ISO 4014:2022 and ASME B18.2.1-2012 (R2021) also describe different product requirements. Legacy DIN 931-1 is withdrawn and was replaced by DIN EN ISO 4014.^{[15] [16]}

Check the thread, head dimensions, grip and thread length, mechanical properties, material restrictions, finish, markings and inspection documents against the drawing. Record the design authority’s approval and identify the accepted alternative by revision or written deviation.

What must the finish specification control?

Specify the coating system and assembly condition. Silver zinc, yellow zinc and black describe appearance; Dacromet is a trade name. Add the coating designation, corrosion acceptance requirement and lubricant or friction condition.

Coating thickness affects thread fit; topcoats and lubricants affect friction during tightening. For electroplated high-strength or surface-hardened steel fasteners, specify the applicable hydrogen-embrittlement controls under GB/T 5267.1-2023 or ASTM F1941/F1941M-16(2025).^{[17] [18]}

Use the designation system of the selected coating standard. GB/T 5267.2-2021 covers zinc-flake coatings. ASTM F3393-24 covers hexavalent-chromium-free zinc-flake systems on inch fasteners from 0.250 in. nominal diameter and metric fasteners from 6 mm.^{[19] [20]} Its scope includes both inch and metric products; select it according to the drawing and application requirements.

For a coated order, include the coating standard and complete designation, corrosion test and acceptance point, controlled appearance, lubricant or friction requirement, and the reports needed at delivery.

Dacromet and Zinc-Flake Coatings for High-Strength Fasteners covers coating selection, hydrogen risk, friction, and corrosion requirements in more detail.

Will the specified bolt work in the actual joint?

Check the mating part as well as the bolt. Match diameter, pitch or TPI and thread tolerance, then confirm the nut’s load capacity. SAE J995, revised July 2017, covers inch-series steel nuts within its scope; GB/T 3098.2-2025 supplies the Chinese nut property requirements.^{[21] [22]}

For a tapped hole, identify the material and required thread engagement. Aluminum, cast iron and inserts need different checks from a standard steel nut, especially where internal-thread stripping controls the design.

Specify washer type and hardness where they matter to clamp-load retention. A dished washer or a soft bearing surface can settle after tightening and reduce preload.

Add temperature, corrosive exposure, cyclic loading, vibration or reuse requirements when they change the selection. Establish tightening instructions for the actual thread, finish, lubricant, bearing surface and mating part.

Request the inspection document when placing the order. ISO 16228:2017 defines declaration F2.1 and test reports F2.2, F3.1 and F3.2. Specify the document type, required test results and manufacturing-lot identification. GB/T 90.1-2023 addresses acceptance inspection, a separate requirement.^{[23] [24]}

What does a complete RFQ description look like?

These two filled examples use a plain, uncoated finish with no added lubricant. They illustrate purchase descriptions, rather than grade or finish recommendations for a particular joint. A coated application needs the full coating requirements described above.

Unified-inch example using ASME and SAE standards

For ASTM A193/A194 service, use the specified bolting and nut grades, classes or conditions, and supplementary requirements in place of this SAE example.

Metric example using GB/T standards

Add 35CrMo to GB/T 3077-2015 when the drawing requires that alloy. Otherwise, let the supplier select a material that meets all requirements of the specified property-class standard.

Is the RFQ ready to send?

Use this checklist to catch missing purchase details:

• Product: bolt, screw or stud; head style; drive; full or partial thread; dimensional standard and edition.
• Size: complete thread designation, tolerance or fit class, nominal length, length datum for nonstandard head forms, and any required thread or grip length.
• Mechanical properties: full property class or grade together with its standard; required head or end marking.
• Material: alloy and material standard only when chemistry or a particular material route is required.
• Finish: coating standard and designation, corrosion acceptance point, color if controlled, friction or lubricant requirement and hydrogen controls where applicable.
• Mating hardware: nut standard and grade, washer type and hardness, or tapped-hole material and engagement requirement.
• Service: temperature, corrosive medium, cyclic loading, vibration, electrical constraints and reuse policy when they affect selection.
• Verification: quantity, drawing revision, inspection document, test report, coating results, lot identification and traceability.

Where a drawing controls the part, put its number and revision on the RFQ and resolve any conflict with the written description before ordering.

Need a specification check?

Send us your drawing or purchase list, including the complete callout, quantity, mating hardware, and service conditions. We will identify any gaps before quotation.

References

1. ASME. ASME B18.24-2023 — Part Identifying Number Code System Standard for B18 Fastener Products. Fastener identification and linkage to the controlling B18 product standard.
2. State Administration for Market Regulation, China. GB/T 1237-2000 — Designation System for Fasteners. Current Chinese fastener designation standard.
3. ASME. ASME B18.2.1-2012 (R2021) — Inch-Series Square and Hex Bolts and Screws. Product geometry and dimensions.
4. State Administration for Market Regulation, China. GB/T 5782-2025 — Fasteners: Hexagon Head Bolts. Current product standard, effective February 1, 2026.
5. ASME. ASME B1.1-2024 — Unified Inch Screw Threads. Thread form, series, class, allowance, tolerance, and designation.
6. SAE International. SAE J429, revised May 2014 — Mechanical and Material Requirements for Externally Threaded Fasteners. Inch-series steel fastener grades within the standard's scope.
7. ASTM International. ASTM A193/A193M-26 — Alloy-Steel and Stainless Steel Bolting for High-Temperature, High-Pressure and Other Special-Purpose Applications. Application-specific bolting material and grade requirements.
8. State Administration for Market Regulation, China. GB/T 5785-2025 — Fasteners: Hexagon Head Bolts with Fine-Pitch Thread. Current fine-pitch product standard, effective February 1, 2026.
9. State Administration for Market Regulation, China. GB/T 193-2003 — General Purpose Metric Screw Threads: General Plan. Diameter and pitch series.
10. State Administration for Market Regulation, China. GB/T 197-2018 — General Purpose Metric Screw Threads: Tolerances. Metric thread tolerance system.
11. State Administration for Market Regulation, China. GB/T 3098.1-2010 — Mechanical Properties of Fasteners: Bolts, Screws and Studs. Current Chinese property-class standard for the products within its scope.
12. ISO. ISO 898-1:2013 — Mechanical Properties of Carbon- and Alloy-Steel Bolts, Screws and Studs. ISO metric property classes, confirmed current in 2025.
13. State Administration for Market Regulation, China. GB/T 3077-2015 — Alloy Structure Steels. Chinese alloy structural steel grades.
14. State Administration for Market Regulation, China. GB/T 3098.6-2023 — Mechanical Properties of Stainless Steel Bolts, Screws and Studs. Stainless fastener grade and property requirements.
15. DIN Media. DIN 931-1:1987-09 — Hexagon Head Bolts with Shank. Withdrawn legacy standard, replaced by DIN EN ISO 4014.
16. ISO. ISO 4014:2022 — Fasteners: Hexagon Head Bolts, Product Grades A and B. Current ISO product standard for coarse-pitch hexagon-head bolts.
17. State Administration for Market Regulation, China. GB/T 5267.1-2023 — Fasteners: Electroplated Coatings. Chinese electroplated fastener coating standard.
18. ASTM International. ASTM F1941/F1941M-16(2025) — Electrodeposited Coatings on Mechanical Fasteners. Application, performance, dimensional, and hydrogen-control requirements.
19. State Administration for Market Regulation, China. GB/T 5267.2-2021 — Fasteners: Non-Electrolytically Applied Zinc-Flake Coatings. Chinese zinc-flake coating standard.
20. ASTM International. ASTM F3393-24 — Zinc-Flake Coating Systems for Fasteners. Unified inch and metric fastener coating requirements within the standard's scope.
21. SAE International. SAE J995, revised July 2017 — Mechanical and Material Requirements for Steel Nuts. Inch-series steel nut grades within the standard's scope.
22. State Administration for Market Regulation, China. GB/T 3098.2-2025 — Mechanical Properties of Fasteners: Nuts. Current Chinese nut property standard, effective February 1, 2026.
23. ISO. ISO 16228:2017 — Fasteners: Types of Inspection Documents. Purchaser-requested declarations and test reports.
24. State Administration for Market Regulation, China. GB/T 90.1-2023 — Fasteners: Acceptance Inspection. Chinese fastener acceptance-inspection standard.
25. ASME. ASME B18.2.2-2022 — Nuts for General Applications. Dimensional product standard for the inch-series hex nut used in the RFQ template.
26. State Administration for Market Regulation, China. GB/T 6170-2015 — Hexagon Nuts, Style 1. Current dimensional product standard for the metric nut used in the RFQ template.