Factory purchase managers call me at least twice a week asking "which grade bolt should I use?" Most of the time they already know — they're calling to double-check, or they're trying to save money by dropping one grade. Here's how I answer, and why dropping a grade is almost never the cost-saving it looks like on paper.
We've been selling Unbrako and other high-tensile fasteners for years. The ₹5 difference between a grade 8.8 bolt and a grade 10.9 bolt has caused more expensive failures than any other single part I can think of.
Reading the head — what the numbers mean
Metric bolt grades use two numbers separated by a dot (4.8, 8.8, 10.9, 12.9). The first number is roughly tensile strength in hundreds of MPa; the second tells you yield as a fraction of tensile.
- 10.9 → ~1,000 MPa tensile, 90% of that as yield (~900 MPa).
- 12.9 → ~1,200 MPa tensile, 90% yield (~1,080 MPa).
- 8.8 → ~800 MPa tensile, 80% yield (~640 MPa).
The marking is stamped on the bolt head. If you can't find a marking, it's probably not a graded bolt — and that itself is useful information.
Grade 4.8 — the hardware-shop tier
General-purpose for light fixtures, low-vibration assemblies, non-structural work. Cheap, widely available, fine when the load is trivial. Not suitable for anything where failure matters — machinery, automotive, structural steel, pressure joints.
Grade 8.8 — the standard industrial choice
This is the default for most industrial fastening. Machine baseplates, pump mounts, gearbox covers, conveyor frames. If the drawing doesn't specify a grade and the load is ordinary, 8.8 is the right answer 90% of the time.
8.8 handles medium tensile load, takes modest preload, and plays nicely with grade 8 nuts. Price-wise, maybe 2× a grade 4.8 bolt of the same size.
Grade 10.9 — when 8.8 isn't enough
Step up to 10.9 when the application has real dynamic load, high preload, or safety-critical function. Automotive suspension bolts, gearbox main mounts, high-pressure flange bolts, die clamps. The bolt costs roughly 30-40% more than 8.8, which is nothing next to the cost of a joint loosening in service.
Caveat: 10.9 bolts need matching grade 10 nuts and often require controlled-torque or yield-point tightening. Don't put a 10.9 bolt in with a grade 5 nut — the nut will strip before the bolt reaches working load.
Grade 12.9 — where Unbrako earns its keep
12.9 is the top of the standard scale. Tensile strength ~1,200 MPa. Used where you need maximum grip force from a small bolt: socket-head cap screws in machine-tool work, gearbox through-bolts on heavy equipment, dies and fixtures.
Unbrako is the brand most engineers specify when the drawing says "grade 12.9" — they've been making these for decades and the quality is consistent. If the application is safety-critical or involves high cyclic load, the extra cost is worth it.
| Grade | Tensile | Yield | Typical use | Relative cost |
|---|---|---|---|---|
| 4.8 | 400 MPa | 320 MPa | Light fixtures, non-structural | 1× |
| 8.8 | 800 MPa | 640 MPa | General industrial — machine bases, pumps, conveyors | 2× |
| 10.9 | 1,000 MPa | 900 MPa | Dynamic load, automotive, gearbox mounts | 2.5–3× |
| 12.9 | 1,200 MPa | 1,080 MPa | Machine-tool SHCS, high-preload joints, dies | 3.5–4× |
What happens when you specify wrong
- Under-specifying. Using an 8.8 where a 10.9 was called for. The bolt stretches plastically under load, loses preload, the joint fatigues, eventually fails. Usually not on day one — three to six months down the line, which makes root cause hard to find.
- Over-specifying. Using a 12.9 where an 8.8 was right. Paid more, and the bolt is actually more brittle — if the joint sees shock loading or hydrogen embrittlement, the 12.9 can fracture where an 8.8 would just yield. More is not always better.
Buying — what to ask for
At our counter, tell me three things and I can pick the right bolt in 30 seconds:
- Size — M-diameter × length (e.g. M12 × 50)
- Grade — 8.8, 10.9, or 12.9 (follow the drawing)
- Finish — plain, black oxide, zinc plated, or stainless
If you're not sure what grade the original bolt was, bring one in or send a clear photo of the head marking to +91-9811104037. Usually I can identify it from the stamp alone and tell you whether the application warrants an upgrade.