ISO 898-2 — nuts with specified property classes and proof load values
How ISO 898-2 nut classes work: proof load values, mating bolt classes, style 1 versus style 2, thin nuts, and what a drawing must state before a nut order is quotable.
| Related service | Heat Treatment |
|---|---|
| Mechanical properties | ISO 898-2 |
| Also written as | ISO 898-2:2022, ISO 898-2:2012 (superseded), ISO 898-6 (merged into Part 2), JIS B 1052-2, ASTM A563 (inch counterpart) |
| Updated |
Scope — and what it does not cover
ISO 898-2 sets the mechanical properties of nuts in carbon and alloy steel with coarse or fine pitch threads roughly M1.6 to M39, in property classes 04, 05, 5, 6, 8, 10 and 12, in styles 1 and 2. The 2012 edition absorbed the former ISO 898-6, so nuts with specified proof load values and nuts with specified hardness now live in one document. It does NOT cover stainless nuts (ISO 3506-2), prevailing-torque locking performance (ISO 2320), welding nuts, or nuts intended for elevated-temperature service.
Standards are copyrighted documents. We explain scope, intent and practical application in our own words; obtain the current edition from ISO, ASTM, DIN or your national standards body.
What actually bites in production
- 1The class number × 100 is approximately the stress the nut must carry under proof load, which is why a class 8 nut is intended for class 8.8 bolts and a class 10 nut for 10.9.
- 2The proof load test is functional: the nut is loaded on a hardened threaded mandrel, must not strip, and must still be removable afterwards. Hardness cannot demonstrate that.
- 3The exact proof stress in the table depends on thread size, style and nut height — style 2 nuts are taller, which is how a lower-hardness nut still reaches the class.
- 4Classes 04 and 05 belong to thin and chamfered nuts and carry substantially reduced load capacity. The number is not comparable with a full-height nut.
- 5The design intent is that the bolt fails before the thread strips. A nut hardened "to match the bolt" defeats that and adds embrittlement risk.
- 6Plating changes both the thread fit and the friction, so the tapping sequence — tapped oversize before plating, or tapped after plating — is part of the specification.
Reference tables
Scroll the table sideways on a narrow screen.
Nut class → proof stress → mating bolt class
| Nut property class | Nominal proof stress (MPa) | Mating bolt class (max) | Typical hardness (HV) |
|---|---|---|---|
| 04 | 380 | — | 188–302 |
| 05 | 500 | — | 272–353 |
| 5 | 500 | 5.8 | 130–302 |
| 6 | 600 | 6.8 | 150–302 |
| 8 | 800 | 8.8 | 180–302 |
| 10 | 1040 | 10.9 | 272–353 |
| 12 | 1150 | 12.9 | 295–353 |
Nominal values. The exact proof stress depends on thread size, nut style (1 or 2) and nut height — read it from the table in the edition in force.
Values on this page are typical industry figures that depend on the part, the coating system and the governing specification. They are not guarantees. Always confirm against the edition in force and the customer drawing — this page does not replace either.
What to write on the drawing or PO
- Nut property class AND style (1 or 2), plus the dimensional standard (for example ISO 4032 regular, ISO 4033 high, ISO 4035 thin). The class is only achievable at the matching height.
- The mating bolt class, so the intended failure mode of the joint is on record.
- Whether proof load testing is required per lot, the sample size, and whether a report is issued with the delivery.
- The coating, and whether the thread is tapped before or after coating, with the oversize allowance if before.
- Any locking or prevailing-torque requirement as a separate line with its own standard.
What goes wrong when it is cited loosely
- A thin nut dimension with a high class written next to it. The height is not there, and no heat treatment will make a thin nut hold a class 10 proof load.
- Asking for the nut to be "as hard as the bolt". Nut hardness is capped deliberately; over-hard nuts are the ones that crack in service.
- Replacing the proof load test with a hardness reading on the nut face. A thread that is hard on the surface and soft below passes hardness and strips under load.
- Ordering plated nuts without stating the tapping sequence, then finding the gauge no longer goes.
Frequently asked questions
Which nut class goes with a 10.9 bolt?+
Class 10, in the style the tables require for that size. The point is that the nut can develop the bolt's proof load without stripping; putting a class 8 nut on a 10.9 joint quietly changes the failure mode.
Do you heat treat nuts?+
Yes. Small nuts are processed in bulk like any other small part and quenched and tempered into the hardness band for the class. Whether the finished nut meets the class is confirmed by proof load testing, which is normally arranged by whoever owns the finished-part specification.
Is a class 8 nut the same as a class 8.8 bolt?+
No. Bolt classes have two numbers, nut classes one. Class 8 nuts are designed to mate with class 8.8 bolts, but the tests behind the two numbers are different.
Values on this page are typical industry figures that depend on the part, the coating system and the governing specification. They are not guarantees. Always confirm against the edition in force and the customer drawing — this page does not replace either.
Related standards
Other standards in this reference
Send the drawing and let us check it against this standard
Send the drawing or the specification line as it is written today, the property class or coating requirement, the thread and tolerance class, and the part itself if you have one. We will tell you which lines are processable as written, which are arithmetically impossible on that thread, and what has to be added before the requirement can be quoted, inspected and defended.
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