Technical Library
The three numbers you need before you can compare two fastener quotes: the pitch, the tensile stress area, and what the property class marking on the head is actually promising.
Coarse pitch is the default and is what you get unless a drawing says otherwise. Fine pitch buys a little more tensile area and better resistance to loosening under vibration, at the cost of being more easily cross-threaded and slower to assemble. The tensile stress area is the figure every strength calculation starts from.
| Size | Coarse pitch (mm) | Fine pitch (mm) | Tensile stress area As (mm²) |
|---|---|---|---|
| M3 | 0.50 | - | 5.03 |
| M4 | 0.70 | - | 8.78 |
| M5 | 0.80 | - | 14.20 |
| M6 | 1.00 | 0.75 | 20.10 |
| M8 | 1.25 | 1.00 | 36.60 |
| M10 | 1.50 | 1.25 / 1.00 | 58.00 |
| M12 | 1.75 | 1.50 / 1.25 | 84.30 |
| M14 | 2.00 | 1.50 | 115.00 |
| M16 | 2.00 | 1.50 | 157.00 |
| M20 | 2.50 | 1.50 | 245.00 |
| M24 | 3.00 | 2.00 | 353.00 |
Stress areas are for coarse pitch. Fine-pitch stress areas are slightly larger; ask if you need them for a specific size.
On a steel bolt the marking is two numbers separated by a dot. The first is the nominal tensile strength in hundreds of MPa. The second is ten times the ratio of yield to tensile strength. So 8.8 is roughly 800 MPa tensile with yield at about 80% of that. It is a specification of the finished, heat-treated fastener, not of the bar it came from.
Stainless is marked differently. A2 and A4 identify the steel group (austenitic, A4 being the molybdenum-bearing grade), and the number after the dash is the tensile strength in tens of MPa. A4-80 is a molybdenum stainless at 800 MPa minimum.
| Class | Tensile strength Rm min (MPa) | Proof / yield Rp0.2 min (MPa) |
|---|---|---|
| 8.8 | 800 | 640 |
| 10.9 | 1040 | 940 |
| 12.9 | 1220 | 1100 |
| A2-70 / A4-70 | 700 | 450 |
| A2-80 / A4-80 | 800 | 600 |
Values above are the common minima for sizes up to M16 (steel) and M20 (stainless). Larger diameters and the cold-worked stainless classes differ - check the governing standard for the size you are specifying.
The trap worth knowing. A2-70 stainless has a yield strength of about 450 MPa, well under a plain 8.8 steel bolt at 640 MPa. Swapping a zinc-plated 8.8 for "the stainless equivalent" to solve a corrosion problem quietly removes about a third of the joint's clamp capacity. If the joint was designed at 8.8, it needs re-checking.
How to use these tables. The values here are standard reference figures published to help you specify and compare parts. They are guidance, not a substitute for your own engineering standard, and several of them (torque above all) depend on assumptions about friction, lubrication and joint material that will differ in your application. Where a number matters, confirm it against the governing specification and against the part drawing. We will supply the plant's actual test data with any quotation.