Bolt & Nut Selection Guide | Grade, Material, Thread, Finish and Locking in One Page
A complete inquiry states five things: grade, material, thread, finish and locking method. This guide explains how grade numbers read, how to choose between 304 / 316 / zinc-plated carbon steel, the trade-offs of coarse vs fine thread and locking options, and lists the five most common selection mistakes.
1. Reading Strength Grades
A carbon steel grade like 8.8 reads: first digit 8 = 800 MPa nominal tensile; the .8 = yield ratio 0.8 (640 MPa yield). The higher the grade, the harder - and the more brittle.
Grade 4.8: light-duty use - furniture, ordinary brackets.
Grade 8.8: the most common "high-strength" grade - machinery, steel structures, automotive.
Grades 10.9 / 12.9: high load, vibration, dies and hydraulics - quenched and tempered alloy steel required.
Stainless A2-70: 700 MPa tensile - slightly below 8.8 in strength but corrosion resistant.
| Grade | Tensile (MPa) | Yield (MPa) | Typical Use |
|---|---|---|---|
| 8.8 | 800 | 640 | General machinery, steel structures |
| 10.9 | 1000 | 940 | High load, vibration service |
| 12.9 | 1200 | 1100 | Tooling, hydraulics |
| A2-70 (304 stainless) | 700 | 450 | Corrosion-resistant, general environment |
| A4-80 (316 stainless) | 800 | 600 | Marine / chemical, strong corrosion |
2. 304 or 316 - or Zinc-Plated Carbon Steel?
Rule of thumb: zinc-plated carbon steel is enough in ordinary environments; go straight to 316 at the seaside or around chemicals; when you need both high strength and corrosion resistance, choose Dacromet-coated carbon steel rather than stainless - 316 cannot reach 10.9-level strength.
| Material | Corrosion Resistance | Strength | Cost | Typical Scenario |
|---|---|---|---|---|
| Carbon steel + zinc | Fair (48-96 h salt spray) | High (up to 10.9) | Low | Indoor machinery, steelwork |
| Carbon steel + Dacromet | Excellent (500 h+ salt spray) | High | Medium | Chassis, outdoor high-strength parts |
| 304 / A2 | Good | Medium (A2-70) | Medium | Food equipment, mild outdoor |
| 316 / A4 | Excellent (chloride-resistant) | Medium (A4-80) | High | Seaside, chemical plants, marine |
3. Coarse vs Fine Thread and Locking Options
Coarse vs fine: at the same size, fine pitch self-locks better and engages more thread turns, but demands more careful assembly and cross-threads easily. Default to coarse for ordinary structures; choose fine for thin walls, precise adjustment and heavy vibration. Always state the pitch - write M10 x 1.25 (fine), not just M10.
Nylon-insert lock nut (DIN985): repeatedly reusable, -40 to +120°C, the first choice for vibrating service.
All-metal lock nut (DIN980): heat resistant, for engines and hot piping, limited remakes.
Flange bolts / nuts: large bearing face plus locking teeth, save the washer.
Threadlocker: most reliable for permanent joints; removal needs heat.
4. The Five Most Common Selection Mistakes
1. Specifying only the size without grade and material - the same M10 bolt can differ 3x in price.
2. Treating stainless as "high strength" - A2-70 is only 700 MPa; substituting it for 10.9 causes safety failures.
3. Using a nut below the bolt's grade - the internal thread strips first and the joint dies.
4. Facing strong vibration with plain nuts plus spring washers - switch to lock nuts.
5. Nylon-insert lock nuts in high-temperature service - the nylon ages and fails; use all-metal locking.
Frequently Asked Questions
Five things: strength grade, material, thread form (coarse/fine and pitch), finish, and locking method - e.g. 'M10 x 1.25 fine, 8.8, DIN 933, Dacromet, with DIN 985 nylon lock nuts'. With all five, quotations come back fast and accurate.
No. Standard 316/A4 fasteners are graded A4-70 / A4-80 (700/800 MPa tensile), below 10.9's 1000 MPa. When you need both high strength and corrosion resistance, the right route is quenched-and-tempered alloy steel with a hydrogen-free coating such as Dacromet, not stainless.
Not recommended. The pairing rule is nut grade at or above the bolt grade (8.8 with grade 8, 10.9 with grade 10, 12.9 with grade 12) - otherwise the weaker internal thread strips first and the joint fails.
Need specific data from Bolt & Nut Selection Guide | Grade, Material, Thread, Finish and Locking in One Page?
Submit your duty parameters (temperature, pressure, media, speed) — engineers reply with grade selection and quotation within 24 hours.