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Bolt torque, grades and vehicle specs, sourced

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Bolt grade identification by head markings (SAE, metric, ASTM)

Tap the marking you see on the bolt head to get its grade, its strength and a link to its torque chart. Three radial lines mean SAE Grade 5, six mean Grade 8, and a stamped number such as 10.9 is a metric property class.

By the Torque Reference editors 4 sources cited Last reviewed Method
Contents6 sections

Tap the marking you see on the bolt head.

Choose a marking to see the grade, its strength and its torque chart.

Key takeaways

  • SAE grades are marked with radial lines: none for Grade 2, three for Grade 5, six for Grade 8 ⁠1.
  • Metric classes are stamped as numbers. The first number is a hundredth of the nominal tensile strength in MPa; the second is ten times the yield-to-tensile ratio ⁠2.
  • Class 8.8 and Grade 5 have nearly the same proof stress (580 MPa is 84.1 ksi against 85 ksi), as do class 10.9 and Grade 8 (830 MPa is 120.4 ksi against 120 ksi).
  • A bolt with no marks should be torqued as Grade 2 or not at all. Guessing high can push it past proof load.

Quick facts

No marksSAE Grade 2 (or ungraded)
3 radial linesSAE Grade 5
6 radial linesSAE Grade 8
Stamped 8.8 / 10.9 / 12.9ISO 898-1 property class
Stamped A325 / A490ASTM F3125 structural bolt

How it works

Each button stands for a head marking defined in the bolt’s standard. The result names the grade, gives its strength, and links to the matching torque chart.

SAE J429 (inch bolts). Marked with radial lines on the head. Grade 5 has three lines 120° apart and Grade 8 has six lines 60° apart. Grades 1 and 2 carry no grade mark ⁠1. Counting the lines and adding two gives the grade number.

ISO 898-1 (metric bolts). Marked with the property class number. In “10.9”, the 10 means a nominal tensile strength of 1,000 MPa and the .9 means the nominal yield is 90% of that. ISO’s own example: 800 MPa nominal tensile with a 0.8 yield ratio is class 8.8 ⁠2. A leading zero, as in 08.8, marks a fastener with reduced loadability because of its geometry ⁠2, so the regular 8.8 torque does not apply.

ASTM F3125 (structural bolts). Heavy hex structural bolts are stamped with the grade, such as A325 or A490, plus the maker’s mark ⁠3. F3125 consolidates the old A325, A490, F1852 and F2280 standards ⁠4.

What each marking means
Marking Grade Proof stress Minimum tensile
None SAE Grade 2, 1/4 to 3/4 in 55,000 psi 74,000 psi
None SAE Grade 2, 7/8 to 1-1/2 in 33,000 psi 60,000 psi
3 radial lines SAE Grade 5, 1/4 to 1 in 85,000 psi 120,000 psi
6 radial lines SAE Grade 8 120,000 psi 150,000 psi
4.6 ISO class 4.6 225 MPa 400 MPa
8.8 ISO class 8.8, up to M16 580 MPa 800 MPa
10.9 ISO class 10.9 830 MPa 1,040 MPa
12.9 ISO class 12.9 970 MPa 1,220 MPa
A325 F3125 Grade A325 – 120 ksi
A490 F3125 Grade A490 – 150 ksi

Scroll the table sideways to see every column.

SAE values are from the Fastenal guide’s reproduction of SAE J429 ⁠1, metric values from ISO 898-1 ⁠2, structural tensile strengths from the RCSC Specification ⁠3.

Worked example

You pull a 3/8-16 bolt from a bracket and the head has six radial lines.

  1. Identify

    Six lines is SAE Grade 8, 120,000 psi proof stress.

  2. Torque

    In the bolt torque calculator, 3/8-16 Grade 8, plain dry, gives 44 ft-lb (59 N·m) for about 6,980 lbf of clamp load.

  3. Compare

    The same bolt as Grade 5 would be 31 ft-lb (42 N·m), and as Grade 2 only 20 ft-lb (27 N·m). Grade 8 carries about 41% more torque than Grade 5 at the same size, because torque scales directly with proof stress (120 ÷ 85).

Reading the result

The grade sets the clamp load the bolt can take. It does not tell you the thread condition, so you still pick dry, zinc, oiled or anti-seize in the torque calculator.

The rough equivalences help when you mix systems. Class 8.8 up to M16 has a proof stress of 580 MPa, which is 84.1 ksi, close to Grade 5’s 85 ksi. Class 10.9 at 830 MPa is 120.4 ksi, close to Grade 8’s 120 ksi. Above M16, class 8.8 rises to 600 MPa ⁠2.

Structural bolts are a different case. A325 and A490 bolts in pretensioned joints are installed by an RCSC method such as turn-of-nut, and the RCSC forbids torque values taken from tables ⁠3. The result links to the A325 and A490 pages instead of a torque chart.

Common mistakes

  • Assuming high. Torquing a Grade 5 bolt to a Grade 8 value loads it to about 106% of its proof load (75% × 120 ÷ 85), with no margin left for friction scatter.
  • Counting raised lettering as lines. Many heads also carry the maker’s mark or initials. Only the radial lines count for SAE grade.
  • Reading 8.8 as 88 or 10.9 as 109. The dot matters: it separates the tensile number from the yield ratio.
  • Ignoring the leading zero. 08.8 means reduced loadability ⁠2.
  • Trusting an unmarked bolt. No marks can mean Grade 2 or an ungraded bolt of unknown strength. If the joint matters, replace it with a marked bolt of the grade the equipment calls for.

Grade-by-grade torque tables are in the grades section.

Sources

  1. 1
  2. 2
  3. 3
  4. 4

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