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

You have a bolt callout on a drawing or an old part in hand. What does a supplier need to reproduce it without guessing? The answer is more than diameter and grade. A purchase-ready specification identifies the product and dimensional standard, the complete thread and length, the required mechanical properties, the finish, the mating hardware, and the evidence that must accompany the lot.

This guide shows how to turn a partial callout such as 1/2-13 UNC-2A × 3 in, SAE Grade 8 or M12 × 1.75 × 80, 10.9 into a complete RFQ. It treats U.S. Unified-inch practice first and the Chinese GB/T metric route afterward, because the two systems express similar engineering requirements in different ways. They should be compared field by field, not blended into a hybrid callout.

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.

If the question is broader—“What should the complete bolt description say?”—continue below. Work in this order: identify the product standard, complete the thread and length, verify strength and material, define the finish, check the mating joint, and specify inspection evidence. The aim is to remove the decisions a supplier would otherwise have to infer.

What makes a bolt specification complete?

1/2-13 gives the nominal diameter and TPI of a Unified thread; M12 gives only the nominal diameter of a metric thread. Neither callout identifies the product form, length, mechanical grade, finish, or inspection level.

A complete specification answers eight separate questions:

No single grade or size code answers all eight questions. Each layer remains governed by its own source standard. ASME B18.24 and GB/T 1237 provide designation systems that point to the applicable product, thread, property, and coating standards.^{[1] [2]}

Length also depends on product form. For the hex-head products used in this guide, nominal length runs from the bearing surface under the head to the end. The product standard also defines the usual thread length for full- and partial-thread products. Other head forms can use a different datum. If the joint needs a specific unthreaded grip, usable thread length, or engagement, state that dimension instead of relying on nominal length alone.^{[3] [4]}

The practical test is simple: could two suppliers read the description and independently produce the same interchangeable part? If product form, thread fit, grip, strength, coating, or inspection is still open to interpretation, the specification is not finished.

What does a Unified-inch drawing require?

On a Unified-inch drawing, identify the product standard, thread, grade, and finish separately. Consider:

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

Use ASME B1.1-2024 for the Unified thread, ASME B18.2.1-2012 (R2021) for the applicable inch-series bolt geometry, and SAE J429, revised May 2014, for Grade 5 or Grade 8 mechanical and material requirements. Naming any one of them does not cover the other two.^{[3] [5] [6]}

Keep the complete thread callout on the drawing and purchase order. A 1/2-13 UNC bolt requires a different mating thread from a 1/2-20 UNF bolt even though both have the same nominal diameter. The same principle applies to numbered machine-screw sizes: in #10-24 UNC, #10 is a diameter designation and 24 is the TPI.

SAE Grade 5 and Grade 8 belong to SAE J429. Structural bolting, pressure equipment, and other services can invoke different ASTM or project specifications. ASTM A193/A193M-26, for example, covers alloy- and stainless-steel bolting for high-temperature, high-pressure, and other special-purpose applications. Its B7 and B8 designations belong to that application-specific system rather than a strength ladder with SAE Grade 8.^[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?

An ISO metric screw thread is a thread system; by itself, it does not identify a Chinese product standard. A bolt can have that thread while its finished dimensions are controlled by GB/T, ISO, DIN, or another product standard. On a Chinese drawing, GB/T 5782-2025 identifies a coarse-pitch hexagon-head bolt product. A fine-pitch hexagon-head bolt follows GB/T 5785-2025 or another explicitly stated fine-pitch product standard.^{[4] [8]}

Consider this purchase description:

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 threads state pitch as a distance in millimeters. M12 × 1.75 and M12 × 1.25 have the same nominal diameter but require different tapped holes or nuts. Do not select the mating thread from M12 alone. Confirm the pitch permitted by the named product standard and state it whenever ambiguity is possible; GB/T 193-2003 supplies the general-purpose metric diameter and pitch series.^[9]

In many standard metric product designations, the coarse pitch is implicit. State the pitch when replacing an unidentified part, transferring a callout between standards systems, or specifying a fine thread.

Metric thread tolerance uses a number and a letter rather than the Unified 2A and 2B notation. For example, M12 × 1.75-6g identifies an external metric thread tolerance class; 6H is a common internal class. GB/T 197-2018 controls the Chinese general-purpose metric thread tolerance system.^[10] The tolerance class controls fit and allowable thread dimensions. Strength is specified separately.

Does the property class also specify the steel?

Class 10.9 controls the finished fastener's mechanical properties, not a single steel chemistry. The same distinction applies to classes 8.8 and 12.9. GB/T 3098.1-2010 supplies the Chinese requirements for these carbon- and alloy-steel fasteners; ISO 898-1:2013 governs the related ISO route within its scope.^{[11] [12]}

The property class does not identify one mandatory steel chemistry. 35CrMo to GB/T 3077-2015 is a material callout; 10.9 to GB/T 3098.1-2010 is a finished-fastener performance callout. A drawing may require both, but naming 35CrMo alone does not prove that the manufactured and heat-treated bolt meets class 10.9.^[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.

Stainless steel follows another path. A2-70 combines stainless steel grade A2 with property class 70; it is not synonymous with Type 304. GB/T 3098.6-2023 covers mechanical properties of stainless steel bolts, screws, and studs in the Chinese standards system. If the drawing requires a specific chemistry as well as a fastener property class, state both requirements with their governing standards.^[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?

No. Before approving a substitution, identify what each designation controls:

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; the applicable class or condition must also be specified. Titanium Grade 5 is Ti-6Al-4V. Neither designation is related to the similarly numbered SAE grades.

Similar numbers or shapes do not establish equivalence. Metric class 10.9 and SAE Grade 8 occupy similar strength ranges, but their threads, dimensions, markings, and governing standards differ. Likewise, GB/T 5782-2025, ISO 4014:2022, legacy DIN 931 callouts, and ASME B18.2.1-2012 (R2021) can point toward a familiar partially threaded hex-head form with different dimensions. DIN 931 is withdrawn; new German or European specifications should use the applicable edition of DIN EN ISO 4014.^{[15] [16]}

Treat a cross-system substitute as an engineering change. Compare the thread and mating parts; head, body, grip, and thread dimensions; mechanical requirements; material and heat-treatment restrictions; finish and friction condition; markings; and inspection documents. Record the comparison against the controlling drawing, obtain approval from the design authority or customer, and identify the approved alternative by drawing revision or written deviation.

What must the finish specification control?

It must control performance and assembly behavior, not just color. Terms such as silver zinc, yellow zinc, and black describe appearance. Dacromet is a trade name. None supplies a complete coating designation, thickness, conversion layer or topcoat, corrosion requirement, friction condition, or hydrogen controls.

These details affect fit and preload: coating buildup consumes thread allowance, while lubricant and topcoat alter thread and bearing friction. Electroplated high-strength or surface-hardened steel fasteners also require hydrogen-embrittlement controls. GB/T 5267.1-2023 and ASTM F1941/F1941M-16(2025) cover electrodeposited fastener coatings in their respective systems.^{[17] [18]}

Choose the zinc-flake coating standard that belongs on the order. GB/T 5267.2-2021 includes designation fields such as hexavalent-chromium status and corrosion-resistance duration. ASTM F3393-24 covers systems without hexavalent chromium on Unified-inch fasteners at least 0.250 in. in nominal diameter and metric fasteners at least 6 mm.^{[19] [20]} Do not copy an ASTM coating code onto a GB/T order, or vice versa; use the designation required by the named coating standard.

State the coating standard and designation, corrosion performance, friction or lubricant condition, appearance, and required tests. Verify the mechanical grade from the marking and certification separately from the coating.

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?

Not necessarily. A bolt can meet every line of its own certificate and still be wrong for the joint. The nut or tapped hole must match the diameter, pitch or TPI, thread system, and internal-thread tolerance class. Its proof-load capability must also suit the external fastener. SAE J995, revised July 2017, covers steel nut grades for automotive and related inch-series applications within its scope; GB/T 3098.2-2025 supplies mechanical-property requirements for nuts in the current Chinese standards route.^{[21] [22]}

Tapped holes add another variable because the internal thread may be aluminum, cast iron, steel, plastic, or an insert rather than a standard nut. State the mating material and required engagement when thread stripping is a design concern. Higher bolt strength cannot compensate for a short or damaged internal thread.

The washer and bearing surface also matter when preload rises. A washer that dishes or a soft clamped surface that embeds can reduce retained clamp load even though the bolt remains intact. Specify washer type and hardness where the joint design depends on them.

Service conditions decide which details need tighter control. Identify temperature, corrosive exposure, cyclic or impact loading, vibration, electrical requirements, and planned reuse where relevant. Property class supplies mechanical data under standard test conditions. Any torque or tightening instruction used to control preload should be established or validated for the actual thread, finish, lubricant, bearing surface, and mating part.

Inspection requirements belong on the order. Choose the document level according to project risk and customer requirements; certificate required is not specific enough. ISO 16228:2017 defines purchaser-requested fastener documents: declaration of compliance F2.1 and test reports F2.2, F3.1, and F3.2. GB/T 90.1-2023 addresses acceptance inspection; it does not select the document that accompanies a shipment.^{[23] [24]} State the acceptance plan and the exact document type or report content, including any required material traceability, coating results, and dimensional inspection.

What does a complete RFQ description look like?

These templates show how the requirements fit together. Complete every bracketed field from the drawing or engineering specification before issuing the RFQ; the examples are not design recommendations.

Unified-inch example using ASME and SAE standards

For pressure, high-temperature, or other service governed by ASTM A193/A194, replace the generic SAE route with the applicable bolting grade, nut grade, class or condition, and supplementary requirements.

Metric example using GB/T standards

Add 35CrMo to GB/T 3077-2015 only when the drawing or customer specification controls chemistry. If chemistry is not a design requirement, specifying class 10.9 alone avoids unnecessarily restricting suppliers.

Is the RFQ ready to send?

Before sending an inquiry, check that the specification answers each applicable item:

• 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.

If an existing drawing controls the joint, cite its number and revision first, then use this checklist only to fill missing purchase details.

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.

From the Catalog

Products covered in this guide

35CrMo Alloy Steel Hex Head Bolt UNC Black Oxide Grade 8 ASME B18.2.1 View Product → Dacromet Grade 12.9 Hexagon Socket Cap Screws M16/M18/M20/M27 Alloy Steel Bolts DIN 912 Gb70.1 Compliant View Product → Steel Hex Nut Grade 5 & Grade 8 UNC ASME B18.2.2 Black Oxide View Product →