As a general rule, the nut should be the same property class as the bolt — or slightly higher. That means a class 10 nut can usually be used with a class 8 bolt, but not the other way around.
1. Basic rule: the nut must not be weaker than the bolt
In a bolted joint, the nut and bolt work together as a system. For the connection to be safe, the nut’s strength (property class) has to be compatible with, and usually at least equal to, the bolt’s strength.
That’s why, in most design guides, you will see a simple rule of thumb:
- Class 8 bolt → use a class 8 nut or higher.
- Class 10 bolt → use a class 10 nut (or the nut recommended by the standard).
2. Is it OK to use a class 10 nut on a class 8 bolt?
Yes, in most cases it is acceptable to use a stronger nut (class 10) with a slightly weaker bolt (class 8), as long as:
- The thread size and pitch are identical (for example M8×1.25 on both).
- You follow the recommended tightening torque for the bolt grade, not the higher nut grade.
- You stay within the limits of the relevant standard (ISO/EN/SAE) for that combination.
The critical case to avoid is the opposite: a weaker nut on a stronger bolt. For example, a mild steel nut or a class 8 nut on a class 10 bolt can fail prematurely, because the nut thread strips before the bolt reaches its design preload.
3. Why matching grades matters
The property class (grade) defines:
- Tensile strength and proof load.
- Hardness of the nut and bolt material.
- How much clamping force the joint can safely carry.
If the nut is too soft compared to the bolt, its threads can strip under load. Even if the joint “feels” tight during assembly, it may not develop the proper preload, which reduces fatigue life and can lead to loosening or failure in service.
4. How to identify bolt and nut classes
Property classes are usually marked directly on the fastener:
- Bolts: class is stamped on the head (e.g. 8.8, 10.9).
- Nuts: depending on the standard, markings may be on one face or on the flats (e.g. “8” or “10”), or indicated by specific symbols in some SAE/ANSI systems.
If there is any doubt about the nut you have in stock, check the manufacturer’s datasheet or ask your supplier to confirm the property class before you mix grades in a safety-critical joint.
5. Practical takeaway
For most industrial applications, you can keep it simple:
- Match the nut to the same class as the bolt wherever possible.
- Using a stronger nut (class 10) with a class 8 bolt is normally acceptable.
- Avoid using a weaker nut on a stronger bolt — this can compromise safety and reliability.
When the joint is critical (high loads, dynamic loading, safety-related components), always follow the relevant ISO/EN/SAE standard and your company’s engineering guidelines.