Bolts of the same size can look alike and still differ in strength: a 10.9 bolt has two and a half times the tensile strength of a 4.6 bolt. The head mark shows the class or grade and the maker, so you can check on site that the bolt matches the drawing and the nut matches the bolt.
Metric steel bolts carry a property class from ISO 898-1, adopted in India as IS 1367 (Part 3). The class has two numbers:
So 10.9 means 1000 MPa tensile and 900 MPa yield. Classes 4.6 and 5.6 are mild steel. Classes 8.8, 10.9 and 12.9 are high tensile steel, hardened and tempered. The dot may be left out, so 88 means 8.8.
ISO 898-1 requires the class and the maker’s mark on every hex head bolt from M5 up, 4.6 included, and socket head cap screw from M5 up, on the top or side of the head. A zero in front, such as 08.8, means reduced loadability: 8.8 steel, but a shape, such as a low head, that cannot carry the full 8.8 load.
Steel nuts to ISO 898-2 carry one number, the nut class, on the face, side or chamfer. It matches the first number of the strongest bolt class the nut is made for:
A higher class nut can replace a lower one, but never the reverse: a class 8 nut on a 10.9 bolt can strip before the bolt takes its full load.
Stainless bolts are marked to ISO 3506-1 with a steel grade, a hyphen and a strength class:
Hex head bolts and socket head screws from M5 up must show the grade, class and maker’s mark. Use an A2 nut on an A2 bolt and an A4 nut on an A4 bolt, of the same or a higher class. SS 202 is not covered by ISO 3506, so it should not carry an A2 or A4 mark. See SS 304 vs SS 316 to choose.
Studs for pipe flanges and pressure work are made to ASTM A193, with the grade and maker’s mark stamped on one end. Their ASTM A194 nuts carry their own grade:
Strain hardened Class 2 B8 and B8M studs carry an extra mark, such as a line under the grade.
Inch bolts to the American SAE J429 standard use lines: three radial lines mean grade 5 and six mean grade 8. Preloaded structural bolting sets to EN 14399 add HR or HV to the class, for example HV 10.9 on the bolt and HV 10 on the nut. Do not mix parts from different sets.
| Mark | Material | What it means | Matching nut |
|---|---|---|---|
| 4.6 | Mild steel | 400 / 240 MPa | Class 5 |
| 5.6 | Mild steel | 500 / 300 MPa | Class 5 |
| 8.8 | Hardened carbon steel | 800 / 640 MPa | Class 8 |
| 10.9 | Hardened carbon or alloy steel | 1000 / 900 MPa | Class 10 |
| 12.9 | Hardened alloy steel | 1200 / 1080 MPa | Class 12 |
| A2-70 | SS 304 family | 700 / 450 MPa | A2-70 |
| A4-70 | SS 316 family | 700 / 450 MPa | A4-70 |
| A4-80 | SS 316 family | 800 / 600 MPa | A4-80 |
| B7 | Cr-Mo alloy steel stud | ASTM A193, hardened | A194 2H |
| B8 | SS 304 stud | ASTM A193 | A194 8 |
| B8M | SS 316 stud | ASTM A193 | A194 8M |
How to read the chart: MPa values are nominal tensile / yield strength (minimum for stainless). ISO 898-1 sets the exact minimum for each size (see the bolt grade chart).
A hex bolt from M5 up with no class mark does not meet ISO 898-1 or ISO 3506-1, so treat it as low strength. Marks can also be faked: 10.9 stamped on soft steel looks real. Buy from a known supplier, check for both the class and the maker’s mark, and ask for the grade in writing on the quote and invoice.
JMIKS Fasteners supplies hex bolts, hex nuts and B7 and B8 studs in SS 304, SS 316, SS 202, mild steel 4.6 and 5.6, high tensile 8.8, 10.9 and 12.9, and B7, B8 and B8M, from M3 to M36 and 1/4 inch to 1-1/4 inch BSW. Mild steel, 8.8 and 10.9 parts come self colour, zinc plated or hot dip galvanized. 12.9 is normally self colour.
It is the ISO 898-1 property class. The first 8 means 800 MPa nominal tensile strength. The second 8 means the yield strength is 80 % of that: 640 MPa.
A class 10 nut, marked 10. A class 12 nut also works. A class 8 nut is too weak and its thread can strip.
Both have 700 MPa minimum tensile strength. A2 is the SS 304 family. A4 is the SS 316 family, which resists sea air and chlorides better.
An ASTM A194 grade 2H nut, marked 2H. B8 studs take grade 8 nuts and B8M studs take grade 8M nuts.
Not where a grade is specified. Hex bolts from M5 up must be marked, so an unmarked one is not proven to any class.
JMIKS Fasteners is a fastener wholesaler and supplier in Nava Odhav, Ahmedabad. Send the size, grade, finish and quantity on WhatsApp (+91 90333 11425) or through the enquiry form, or get our catalogue from the catalogue page. We reply within one working day.
Send the size, grade, finish and quantity on WhatsApp. We reply with the rate and stock position within one working day.
