Fastener Industry Standards Guide
Overview of ANSI, ISO, SAE, ASTM and other fastener standards and what they mean for real-world applications.
Quick Navigation
Why Standards Matter
Consistency, safety & interoperability
Measurement Systems
Inch vs metric, thread series & classes
Standards Organizations
ANSI, ISO, SAE, ASTM, IFI & more
Strength Grades
SAE grades & metric property classes
Materials & Finishes
Common specs for material & coating
Head Markings
How to read bolt & screw markings
Why Fastener Standards Matter
Fastener industry standards define dimensions, strength levels, materials, threads and performance requirements so components from different manufacturers work together safely and consistently.
Understanding the basics of common standards helps you match the right bolt, screw or nut to your application and interpret the information printed on packaging, drawings and certifications.
What Industry Standards Do
- Define dimensions: Head sizes, thread pitch, tolerances and lengths.
- Set performance levels: Minimum tensile, proof and yield strengths.
- Specify materials: Steel grades, stainless alloys and corrosion resistance.
- Standardize terminology: Common language for engineers, buyers and installers.
- Improve interchangeability: Parts from different brands that fit and perform similarly.
Important Disclaimer
- This guide is a simplified reference and is not a substitute for full standards documents.
- Always follow the specifications on your drawings, prints or project requirements.
- For critical or structural applications, consult a qualified engineer.
Download Fastener Industry Standards Quick Reference
Printable one-page summary of common fastener standards, strength grades and thread series.
Download Industry Standards Guide (PDF)Measurement Systems & Thread Series
Fasteners are produced in two main measurement systems: inch (imperial) and metric. Each system uses its own standards for thread series, pitch and tolerances.
Inch (Imperial) Thread Series
| Thread Series | Abbreviation | Description | Typical Use |
|---|---|---|---|
| Unified Coarse | UNC | Fewer, deeper threads per inch for strength in softer materials. | General-purpose bolts, nuts and screws. |
| Unified Fine | UNF | More threads per inch for better adjustment and higher tensile area. | Automotive, machinery, vibration-prone assemblies. |
| Unified Extra Fine | UNEF | Very fine pitch where small adjustments or short engagement are required. | Instrumentation, aerospace and specialty applications. |
Metric Thread Series
| Thread Type | Designation Example | Description | Typical Use |
|---|---|---|---|
| Metric Coarse | M10 × 1.5 | Standard pitch for most metric fasteners. | General structural and machinery applications. |
| Metric Fine | M10 × 1.25 | Smaller pitch than coarse for finer adjustment and higher tensile area. | Automotive, high-vibration joints, thin materials. |
| Metric Extra Fine | M10 × 1.0 | Very fine pitch for limited engagement length. | Special machinery, precision equipment. |
Thread Tolerance Classes
- Inch (Unified): Common classes are 2A (external) / 2B (internal) for general use and 3A / 3B for high-precision, tight-tolerance applications.
- Metric: External threads often use 6g, internal threads 6H for standard fits. Other class combinations adjust clearance or interference.
- Never mix inch and metric fasteners or hardware in the same joint.
Common Fastener Standards Organizations
Many different organizations publish standards that apply to fasteners. The table below summarizes the most commonly referenced groups and what they cover.
| Acronym | Full Name | Typical Role in Fasteners | Examples of Standards |
|---|---|---|---|
| ANSI | American National Standards Institute | Approves national standards for dimensions and tolerances. | Unified thread forms, dimensional callouts. |
| ASME | American Society of Mechanical Engineers | Publishes mechanical design and dimension standards. | ASME B18 series for bolt, screw and nut dimensions. |
| SAE | Society of Automotive Engineers | Defines mechanical and performance requirements, especially for automotive. | SAE J429 mechanical bolt grades (Grade 2, 5, 8). |
| ASTM | ASTM International | Specifies material, mechanical and test requirements. | ASTM A307, A193, A194, F3125 and many others. |
| ISO | International Organization for Standardization | Develops global metric fastener standards. | ISO 898-1 property classes 8.8, 10.9, 12.9; ISO 4014, 4017, 4762. |
| DIN | Deutsches Institut für Normung | German standards widely used in metric fasteners. | DIN 933 hex bolts, DIN 912 socket cap screws. |
| IFI | Industrial Fasteners Institute | Industry group that publishes technical bulletins and standards. | IFI standards for dimensions, tolerances and testing. |
How Standards Work Together
A single fastener can be covered by several standards at once, such as an ASTM material specification, an ASME dimensional standard and an ISO or SAE mechanical requirement.
Strength Grades & Property Classes
Strength standards define the minimum mechanical properties a fastener must meet, such as tensile strength and proof load. These values are often identified by head markings.
Inch-Series Bolt Grades (SAE / ASTM)
| Grade | Common Standard | Typical Minimum Tensile Strength | Typical Applications |
|---|---|---|---|
| Grade 2 | SAE J429 / ASTM A307 | Low carbon steel for low-strength applications. | Light duty, non-critical joints, general hardware. |
| Grade 5 | SAE J429 | Medium carbon steel, heat-treated. | Automotive, machinery, general structural use. |
| Grade 8 | SAE J429 | High-strength alloy steel, quenched and tempered. | High-clamp-load joints, heavy equipment, severe service. |
| Structural Bolts | ASTM F3125 (A325, A490 series) | High strength with controlled ductility. | Steel building connections, bridges and structural joints. |
Metric Property Classes (ISO 898-1)
| Property Class | Typical Minimum Tensile Strength | Approximate Inch Grade | Typical Applications |
|---|---|---|---|
| 4.6 | Lower-strength carbon steel. | Roughly similar to Grade 2. | Light-duty metric fasteners. |
| 8.8 | Medium carbon steel, heat-treated. | Similar to Grade 5. | General machinery, automotive and structural metric bolts. |
| 10.9 | High-strength alloy steel. | Between Grade 8 and higher structural grades. | High-strength metric joints, suspension, heavy machinery. |
| 12.9 | Very high strength, typically socket cap screws. | High-strength socket products. | Tooling, clamping and critical assemblies. |
Strength Selection Tips
- Match or exceed the grade or property class specified on drawings.
- Do not substitute lower grades without engineering approval.
- Higher strength is not always better; it can change joint behavior and required torque.
Material & Finish Standards
Material and coating standards define the base metal, heat treatment and corrosion protection of a fastener.
Common Material Specifications
- Carbon Steel Bolts: ASTM A307, SAE J429 (Grades 2, 5, 8).
- Alloy Steel Bolts: ASTM A325 / A490 (now F3125), high-strength structural.
- Stainless Steel Fasteners: Grades such as 18-8 (304), 316 and others defined by ASTM and ISO standards.
- High-Temperature Alloys: Specifications like ASTM A193 for studs and bolts in elevated temperature service.
Finish & Coating Specifications
- Electroplated Zinc: Common for indoor corrosion protection; thickness and chromate type are standardized.
- Hot-Dip Galvanizing: Thicker zinc coating for outdoor and structural use.
- Mechanical Galvanizing or Mechanical Zinc: Alternative to hot-dip for threaded assemblies.
- Other Coatings: Mechanical or chemical coatings specified for chemical resistance or torque control.
Material & Finish Considerations
- Always consider environment (indoor, outdoor, chemical, temperature) when choosing materials.
- Do not mix stainless and carbon steel in a way that could cause galvanic corrosion without planning for it.
- Confirm compatibility between coating type and required torque / preload.
Reading Fastener Head Markings
Head markings identify the manufacturer, material grade and sometimes the standard or property class.
| Fastener Type | Typical Marking | Meaning |
|---|---|---|
| Inch Hex Bolt | No radial lines | Usually SAE Grade 2 (low carbon steel) unless otherwise specified. |
| Inch Hex Bolt | Three radial lines | SAE Grade 5 medium carbon steel. |
| Inch Hex Bolt | Six radial lines | SAE Grade 8 alloy steel. |
| Metric Hex Bolt | Number such as 8.8, 10.9, 12.9 | Metric property class per ISO 898-1. |
| All Types | Manufacturer’s symbol | Identifies the producer for traceability and quality control. |
Marking Reminders
- Never grind or remove head markings on structural or critical fasteners.
- If markings do not match your drawing or specification, do not use the fasteners until verified.
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