⚡ Key takeaways
- Five families dominate: GB (China), ISO (international), DIN/EN (Germany and Europe), ANSI/ASME (United States) and JIS (Japan).
- ISO is the international reference. GB and DIN largely adopt ISO verbatim; ANSI and JIS keep their own dimensional systems.
- Metric versus inch is the fundamental divide. M-series and UNC/UNF threads are not interchangeable at any level.
- Property classes are separate languages: 4.6 / 8.8 / 10.9 / 12.9 on the metric side, SAE Grade 2 / 5 / 8 on the inch side.
- Old DIN numbers have been superseded. DIN 912 is now DIN EN ISO 4762, and the year of the edition matters.
- IFI is the Industrial Fasteners Institute, the body behind detailed inch fastener dimensions in the US.
- Quote the standard number, not just the dimensions. The standard already carries dimensions, tolerances, material and marking.
1. A wrongly quoted standard number
2. The five standards families
3. Thread standards
4. Property classes
5. Common number cross-reference
6. Misconceptions
7. Decision tree and FAQ
1. A wrongly quoted standard number
An export order specified "M8 × 20 socket head cap screw" and the buyer asked for DIN 912. The factory shipped to DIN 912 and the goods were rejected: the purchase order technically called for ISO 4762.
The two are almost dimensionally identical, but the buyer inspected against ISO 4762 and the head height and hexagon socket tolerances differ slightly. Enough to fail incoming inspection.
The lesson: write the standard number completely and precisely, including the edition year. A purchase order should read "DIN EN ISO 4762:2015", not "DIN 912 equivalent". The same discipline applies to drawings, where a bare "M8 socket head" leaves the supplier to guess.
2. The five standards families
| Family | Country / body | Thread | Characteristics |
|---|---|---|---|
| GB | China (SAC) | Metric | Largely identical adoption of ISO |
| ISO | International (ISO/TC 2) | Metric | The global baseline |
| DIN / EN | Germany / Europe (CEN) | Metric | The most rigorous in industrial practice |
| ANSI / ASME | United States | Mostly inch | UNC / UNF threads, IFI dimension tables |
| JIS | Japan | Metric | Precise, shorter length series |
The practical hierarchy is that ISO sits in the middle and most national metric systems now mirror it. Where a national system still diverges, it is usually in head across-flats, length increments or the marking requirement rather than in the thread itself.
3. Thread standards
| Standard | What it defines |
|---|---|
| ISO 68-1 | Basic metric thread profile |
| ISO 724 | Metric thread general dimensions |
| ISO 965 | Metric thread tolerances (6g external / 6H internal) |
| ASME B1.1 | Unified inch threads, UNC and UNF |
| JIS B 0205 | JIS metric thread |
For export work, 6g/6H is the default tolerance class and covers the overwhelming majority of commercial fasteners. Anything tighter — 4h6h for a precision fit, for example — needs to be stated explicitly and costs more, because it requires better gauging and more frequent tool changes.
4. Property classes
| Metric class | SAE grade | Tensile strength (MPa) | Yield ratio |
|---|---|---|---|
| 4.6 | Grade 2 | 400 | 0.6 |
| 8.8 | Grade 5 | 800 | 0.8 |
| 10.9 | Grade 8 | 1,040 | 0.9 |
| 12.9 | — | 1,220 | 0.9 |
Metric 8.8 is roughly equivalent to SAE Grade 5 in strength, but equivalent is not interchangeable: the dimensions, thread form and head geometry all differ.
Read the metric class as two numbers. The first is tensile strength in hundreds of MPa; the second is the yield ratio. So 10.9 means 1,040 MPa tensile and a yield of 900 MPa. That convention makes the class self-documenting once you know it, and it is the reason the metric system is easier to specify than SAE grades, whose numbers carry no arithmetic meaning.
5. Common number cross-reference
| Product | DIN | ISO | GB |
|---|---|---|---|
| Socket head cap screw | DIN 912 | ISO 4762 | GB/T 70.1 |
| Hexagon head bolt (full thread) | DIN 933 | ISO 4017 | GB/T 5783 |
| Hexagon head bolt (part thread) | DIN 931 | ISO 4014 | GB/T 5782 |
| Hexagon nut | DIN 934 | ISO 4032 | GB/T 6170 |
| Plain washer | DIN 125 | ISO 7089 | GB/T 97.1 |
| Spring washer | DIN 127 | — | GB/T 93 |
| Socket set screw | DIN 916 | ISO 4026 / 4027 / 4029 | GB/T 78 / 79 / 80 |
Note that spring washers have no ISO equivalent. ISO dropped them because the locking effect is unreliable, and DIN 127 survives mainly because legacy drawings still call it out. If a design relies on a spring washer for vibration security, that reliance should be re-examined rather than standardised.
6. Misconceptions
7. Decision tree and FAQ
Step 1: where is the customer?
China → GB. Europe → DIN/EN. United States → ANSI/ASME, with IFI for detailed dimensions. Japan → JIS.
Step 2: metric or inch thread?
M-series → ISO, GB, DIN or JIS. UNC/UNF → ASME B1.1 and the IFI handbook.
Step 3: quote a number or a dimension?
Quote the number plus the year. Reserve bare dimensions for custom parts that no standard covers.
FAQ
Q: Can DIN 912 and ISO 4762 be interchanged? In most commercial assemblies yes, and the vast majority of parts sold today are dual-marked. For safety-critical or automated assembly, do not interchange without checking head height and socket depth against the drawing.
Q: What is IFI? The Industrial Fasteners Institute, the US trade body. Its Fastener Standards handbook (currently the 5th edition) is the reference for inch fastener dimensions that ASME does not cover in detail.
Q: How does JIS B 1111 differ from ISO 4014? JIS hex heads have a smaller across-flats for the same nominal diameter and a shorter length series, so a 13 mm JIS head may replace a 13 mm ISO head at a different nominal size. Never assume a JIS part is drop-in for an ISO part without a drawing check.
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