Hex bolt grades explained: Grade 2, 5, 8 and stainless
Last updated 2026-09-30
The grade of a bolt tells you how strong it is. Picking the right one matters: too weak and it can stretch or snap; stronger than needed usually just costs more. Where an engineer, equipment maker or code specifies a grade, use exactly that grade.
Inch bolts (SAE J429)
| Grade | Head marking | Typical use |
|---|---|---|
| Grade 2 | No marks | Light-duty, non-critical fastening: furniture, light brackets, general hardware |
| Grade 5 | 3 radial lines | The everyday workhorse: automotive, equipment, machinery, general construction |
| Grade 8 | 6 radial lines | High-strength, high-load joints: suspension, heavy equipment, high-vibration assemblies |
Grade 5 bolts are medium-carbon steel, heat treated; Grade 8 bolts are alloy steel, heat treated for higher strength. Shop hex bolts & cap screws and use the Grade / class filter to narrow the list.
Metric bolts (property class)
Metric bolts are stamped with a property class on the head:
- 8.8: roughly comparable to Grade 5, the common general-purpose class.
- 10.9: roughly comparable to Grade 8, for high-strength joints.
- 12.9: the strongest common class, typical of socket head cap screws.
Stainless steel (18-8)
18-8 (304) stainless resists rust and chemicals and is ideal outdoors, around water and in food areas. It is generally not as strong as Grade 5 or Grade 8 steel, so don't swap stainless in where a high grade is specified. Stainless threads can also seize (gall); an anti-seize compound helps.
Match the nut and washer
- Use a nut of the same grade or higher as the bolt: Grade 5 bolts with Grade 5 nuts, Grade 8 with Grade 8. Find them under nuts.
- SAE flat washers have a smaller outside diameter for a closer fit; USS flat washers are larger and spread load on soft materials like wood. See washers.
Choosing a finish
- Zinc plated (clear or yellow): indoor and mildly damp areas; inexpensive corrosion protection.
- Hot-dip galvanized: outdoors and in wet conditions. Pair with galvanized nuts, which are made to fit the thicker coating.
- Plain (black oxide or oiled): indoor, dry use, or parts that will be painted.
- Stainless: best corrosion resistance, lower strength than Grade 5/8 steel.
Need help?
Tell us the application, size and quantity and we'll help you pick: call (908) 824-0916 or send us a message. See also how to read thread sizes.