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's 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's 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 and loses preload. The joint fatigues, then 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.