PERFECT ENGINEERING COBolt Grades Explained: 8.8 vs 10.9 vs 12.9
The two numbers on a bolt head tell you how strong it is. Choosing the wrong grade is one of the most common reasons joints loosen or bolts snap — here's how we explain it to customers at our counter.
How to read the grade
Metric steel bolts are marked with a property class such as 8.8 under ISO 898-1. The first number × 100 gives the nominal tensile strength in MPa; the second number tells you the yield point as a fraction of that. So a 10.9 bolt has a nominal tensile strength of about 1000 MPa and starts to permanently stretch at about 90% of that (around 900 MPa).
Grade comparison
| Grade | Min. tensile (MPa) | Min. yield (MPa) | Typical use | Pair with nut |
|---|---|---|---|---|
| 4.6 | 400 | 240 | Light-duty, non-critical | Class 4 / 5 |
| 8.8 | 800 | 640 | General machinery & structural | Class 8 |
| 10.9 | 1040 | 940 | High-load & automotive | Class 10 |
| 12.9 | 1220 | 1100 | Allen bolts, dies & tooling | Class 12 |
| A2-70 (SS 304) | 700 | 450 | Corrosion resistance | A2-70 nut |
| A4-80 (SS 316) | 800 | 600 | Marine & chemical | A4-80 nut |
Values are minimums from ISO 898-1 (steel) and ISO 3506-1 (stainless), rounded. 8.8 values shown for sizes up to M16; above M16 the minimums are slightly higher (830 / 660 MPa).
Which grade should you use?
- 8.8 — the default for machinery, frames, brackets and structural steelwork. Easy to source in every size and finish.
- 10.9 — for heavily loaded or vibrating joints: automotive, earth-moving and heavy equipment, flanges and high-preload joints.
- 12.9 — the standard grade for socket head cap screws (allen bolts) in dies, moulds, machine tools and fixtures where a small screw must carry a high load.
- Stainless A2-70 / A4-70 — choose for corrosion resistance (food, pharma, outdoor, marine), not strength. An A2-70 bolt is weaker than an 8.8, so don't swap stainless in for a 10.9 or 12.9 bolt.
Three mistakes we see often
- Wrong nut. Pair the bolt with a nut of matching class — class 8 nut with 8.8, class 10 with 10.9, class 12 with 12.9. A weaker nut strips before the bolt reaches its strength.
- Electroplating 12.9 bolts. Zinc electroplating can cause hydrogen embrittlement in very high-strength steel, leading to sudden cracking. For 10.9 and especially 12.9, prefer black oxide (self colour) or zinc flake (Geomet / Dacromet) coating.
- Soft washers under high-grade bolts. Use hardened washers with 10.9 and 12.9 bolts; ordinary mild-steel washers dish and lose preload.
Common questions
Is 10.9 stronger than 8.8?
Yes. A 10.9 bolt has roughly 30% higher tensile strength and about 45% higher yield strength than an 8.8 bolt of the same size.
Why are allen bolts usually 12.9?
Socket head cap screws have a small head and are often used in compact tooling where high clamping force is needed from a small screw, so manufacturers like TVS/Sundaram Fasteners and Unbrako make them in grade 12.9 as standard.
Can I use a stainless steel bolt instead of 10.9?
Not where strength matters. Stainless A2-70 has a minimum tensile strength of about 700 MPa, much lower than 10.9. Use stainless only when corrosion resistance is the main requirement.
Need help choosing?
Send us the size, grade and quantity on WhatsApp and we'll recommend the right part and quote the same day.
Related: TVS/Sundaram Fasteners Hex Bolts →TVS Allen Bolts →Nylock Nuts →Washers →