Nut Grade & Bolt-Matching Guide
A nut should be at least as strong as the bolt it threads onto — otherwise the nut becomes the weak link and can strip before the bolt reaches its rated load. This guide shows which nut grade pairs with each common bolt grade, in both inch and metric, plus the structural and high-temperature specs.
Bolt-to-Nut Grade Chart
| Bolt | Recommended Nut | Notes |
|---|---|---|
| Grade 2 / low-carbon | Grade 2 hex nut (or A563 Grade A) | General, lower-load work |
| Grade 5 (SAE J429) | Grade 5 hex nut (or A563 Grade B / DH) | Automotive, machinery |
| Grade 8 (SAE J429) | Grade 8 hex nut (or A563 DH) | Heavy-duty, high-strength |
| Grade 9 / L9 alloy | Grade 9 all-metal lock nut or Grade 8 nut | Highest standard strength |
| Metric class 8.8 | Class 8 nut | Metric medium-high strength |
| Metric class 10.9 | Class 10 nut | Metric high strength |
| A325 / F3125 structural | A563 DH heavy hex nut | Steel construction; match finish |
| A490 structural | A563 DH heavy hex nut | High-strength structural |
| A193 B7 stud (flange/high-temp) | A194 Grade 2H heavy hex nut | Piping, pressure, high temperature |
Reference only. Higher-grade nuts may always be substituted for lower (a Grade 8 nut works on a Grade 5 bolt), but never the reverse. For coated/galvanized structural assemblies, the nut finish and lubrication must match the bolt per the connection spec. Design responsibility for any specific application remains with the specifying engineer.
Why Grade Matching Matters
Bolt grades describe tensile strength; nut grades are matched so the nut's stripping strength meets or exceeds the bolt's tensile strength. Put a low-grade nut on a high-grade bolt and the threads can strip before the bolt is fully loaded — a hidden failure mode, because the joint looks fine until it lets go. Matching (or exceeding) the grade keeps the bolt as the controlling element.
How to Tell a Nut's Grade
- Markings: many nuts carry dots, lines, or numbers indicating grade; A563 DH and A194 2H are stamped accordingly.
- Metric: a class number (8, 10, 12) is usually stamped on the nut.
- Heavy vs finished: structural connections use heavy hex; see our size chart for the dimensional difference.
- When in doubt, send us a photo or the bolt spec and we'll identify and match it.
Frequently Asked Questions
What grade nut do I use with a Grade 8 bolt?
A Grade 8 hex nut, or an A563 DH nut. The nut must be at least as strong as the bolt; a Grade 8 (or DH) nut meets that, while a Grade 2 or 5 nut could strip before the Grade 8 bolt reaches its rated load.
What nut goes with an A325 structural bolt?
An A563 DH heavy hex nut is the standard for A325 (and A490) structural bolting. On galvanized assemblies the nut is hot-dip galvanized, over-tapped, and lubricated to match the bolt.
Can I use a higher-grade nut than the bolt?
Yes. A higher-grade nut on a lower-grade bolt is always acceptable — the bolt remains the controlling element. You can never safely go the other way (a lower-grade nut on a higher-grade bolt).
What metric nut class goes with class 10.9 bolts?
A class 10 nut for class 10.9 bolts, and a class 8 nut for class 8.8 bolts. As with inch grades, match or exceed the bolt's strength class.
What is the difference between A563 DH and A194 2H nuts?
A563 DH is the structural-bolting nut for A325/A490 in steel construction. A194 Grade 2H is for high-strength studs such as A193 B7 and high-temperature/pressure flange joints in piping and pressure vessels. Use the spec your drawing calls out.
Need Nuts Matched to Your Bolts?
Send us your bolt grade, size, and finish and we'll match the correct nut — Grade 2/5/8/9, metric class, or structural A563 DH / A194 2H.