Bolt basics
What the numbers on a bolt's head mean
That 8.8 or 10.9 stamped on a bolt is not a size. It is a strength grade. Here is how to read it in two steps, and match your nut to it.
- 8.8800 MPa tensile, 640 MPa yield
- 10.91,000 MPa tensile, 900 MPa yield
- 12.91,200 MPa tensile, 1,080 MPa yield
Most metric bolts carry two numbers and a dot on the head, like 8.8 or 10.9. It is not a size. It is a strength grade, called the property class.
Read it in two steps
- First number × 100 = tensile strengthFor 8.8 that is 800 MPa: the pull the bolt takes before it breaks.
- Second number × 10 = yield, as a percentage of tensileFor 8.8 that is 80% of 800, which is 640 MPa: where the bolt stops springing back.
8 × 100 = 800 MPa tensile (the tall bar)8 × 10 = 80% of 800 = 640 MPa yield (the gold bar)
| Class | Tensile (MPa) | Yield (MPa) | Yield as % of tensile |
|---|---|---|---|
| 4.6 | 400 | 240 | 60% |
| 8.8 | 800 | 640 | 80% |
| 10.9 | 1,000 | 900 | 90% |
| 12.9 | 1,200 | 1,080 | 90% |
Yield: the point of no return
Below yield, a bolt stretches a little and springs back. Past it, it stays stretched. A bolt that has stretched has already failed, it just has not snapped.
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Match the nut
Nuts carry a class too, and they should be paired with the bolt. A class 8 nut goes with an 8.8 bolt, class 10 with 10.9, class 12 with 12.9. Pair them up.
While you are reading the box
| You see | It means |
|---|---|
| M8 | 8 mm nominal thread diameter |
| × 50 | 50 mm long. For hex bolts, measured under the head |
| M8 (plain) | Coarse thread, 1.25 mm pitch. A fine M8 is 1.0 mm pitch and is marked as such |
Next time you hold a bolt, read its head.
Need the right class?
Tell us the size and the class you need, or send a photo of the bolt you are replacing. We will confirm the match before we quote.
Sources
- ISO 898-1: mechanical properties of fasteners, bolts, screws and studs
- ISO 898-2: nut property classes
- ISO 4014 / ISO 4017: hex bolt and screw dimensions, length measured under the head
- ISO 261: general-purpose metric thread pitches
