Mechanical requirements by diameter
Inch grades are published in ksi and metric classes in MPa. The other column on every row is a conversion at 1 ksi = 6.894757 MPa, rounded to the nearest whole unit - it is arithmetic on the published figure, not a second published figure.
The head marking
The class number 8.8 stamped on the head, with the manufacturer's identifier.
The two numbers are the strength, not a serial number. The first times 100 is the nominal tensile strength in MPa; the two multiplied together times 10 is the nominal yield. Class 8.8 is 800 MPa tensile and 640 MPa yield, and you can read that straight off the head. Class 8.8 is the metric workhorse, roughly where SAE Grade 5 sits.
Count the radial lines and add two. That rule identifies exactly two grades - SAE Grade 5 with three lines and SAE Grade 8 with six - and it fails everywhere else. Grades 1, 2 and 4 carry no marking at all, so an unmarked head is not evidence of Grade 2; it is evidence of nothing, and unmarked hardware that meets no standard looks exactly the same. ASTM grades are stamped with the specification (A325, A490, BC, BD, 307A) rather than lines, and metric classes are stamped with the class number. Four marking systems share one bin.
Material and where it is used
Medium carbon steel or carbon steel with additives, quenched and tempered. The default metric structural and automotive class.
The only class in this set whose strength changes with diameter. An M20 class 8.8 bolt is a different specification from an M12 class 8.8 bolt, and the difference is real: 830 MPa tensile against 800.
ISO 898-1 notes that the d less than or equal to 16 mm values do not apply to structural bolting, where the larger-diameter values apply from M12 up.
Frequently asked questions
How strong is a ISO property class 8.8 bolt?
Over d = 1.6 mm through 16 mm, ISO property class 8.8 requires a minimum tensile strength of 800 MPa (116 ksi), a minimum yield of 640 MPa (93 ksi) and a proof load of 580 MPa (84 ksi). The requirement changes in the larger sizes - 2 diameter bands are published in all.
What does a ISO property class 8.8 head marking look like?
The class number 8.8 stamped on the head, with the manufacturer's identifier.
What is a ISO property class 8.8 bolt made of?
Medium carbon steel or carbon steel with additives, quenched and tempered. The default metric structural and automotive class.
Does ISO property class 8.8 get weaker in larger sizes?
The requirement changes with diameter. d = 1.6 mm through 16 mm: 580 MPa (84 ksi) proof; d over 16 mm through 72 mm: 600 MPa (87 ksi) proof. Quoting one figure across the whole range gets part of it wrong.
What this section covers
Twenty-two grades and classes: the SAE J429 inch grades, five ASTM specifications and the ISO 898-1 property classes. It is not every grade in circulation. Stainless classes (ISO 3506 A2 and A4, ASTM F593), the ASTM A193 and A320 high and low temperature grades, ASTM F1554 anchor rod grades and SAE grades 3, 5.1 and 8.1 are not carried here.
Where these figures come from
Dimensions are as defined by ISO 898-1:2013, Table 3. This site is an independent reference and is not affiliated with, endorsed by or approved by any standards body.
Every value on this page was cross-checked against 5 independent published sources: