On metric ISO (International Standards Organization) bolts the numbers on the head refer to; example 8.8, 9.8, 10.9.......




The digits to the left of the decimal point indicate 1/100 of the ultimate tensile strength in Newton’s per square millimetre(N/mm).

The digits to the right of the decimal point indicate the ratio of yield strength to ultimate tensile strength.



Conversion factor;

One unit of stress 1 N/mm is equal to 145.037 psi



So a 10.9 metric ISO bolt would have a minimum ultimate tensile strength of 145,000 psi and a minimum yield strength to ultimate tensile strength of 130,000 psi.



If the numbers are underlined this refers to low carbon martensitic steel



So where does this leave us when referring to SAE standard bolts?



ISO 6.8 = SAE 2, don’t use on your car!

ISO 8.8 = SAE 5 or 120,000 psi minimum

ISO 10.9 = SAE 8 or 150,000 psi minimum

ISO 12.9 = NAS 624-644 National Aerospace Standard or 180,000 psi minimum



Now I don’t want to get into mechanical properties but if you must know 10.9 ISO bolts are generally medium carbon steel with additives such as Boron, Mn or Cr, quenched and tempered. Or alloy steel quenched and tempered.