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
No grade marking. The head carries the manufacturer's identifier and nothing else.
No marking. This is the grade most often assumed when a head is blank, and the assumption is not safe: the same blank head is used for Grade 1.
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
Low or medium carbon steel. General hardware, brackets, light machine assembly.
The 55 ksi band applies only to bolts and screws 6 in long and shorter, and to studs of any length. A longer Grade 2 bolt of the same diameter drops to the Grade 1 requirements - J429 Table 1, footnote 7. Length is not a property of a diameter, so no table can resolve this for you.
Frequently asked questions
How strong is a SAE Grade 2 bolt?
Over 1/4 in through 3/4 in, SAE Grade 2 requires a minimum tensile strength of 74 ksi (510 MPa), a minimum yield of 57 ksi (393 MPa) and a proof load of 55 ksi (379 MPa). The requirement changes in the larger sizes - 2 diameter bands are published in all.
What does a SAE Grade 2 head marking look like?
No grade marking. The head carries the manufacturer's identifier and nothing else.
What is a SAE Grade 2 bolt made of?
Low or medium carbon steel. General hardware, brackets, light machine assembly.
Does SAE Grade 2 get weaker in larger sizes?
The requirement changes with diameter. 1/4 in through 3/4 in: 55 ksi (379 MPa) proof; Over 3/4 in through 1-1/2 in: 33 ksi (228 MPa) 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 SAE J429 (Revised JAN1999), Table 1. 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 7 independent published sources:
- SAE J429 Revised JAN1999, Tables 1 and 2 (mechanical requirements, chemistry and identification marking) - SAE International (copy hosted by Finesz), Revised JAN1999 (read 2026-08-27)
- SAE J429 specification - Boltport Fasteners, edition not stated (read 2026-08-27)
- Screw - head markings and properties for inch-system and metric hex-head cap screws - Wikipedia, revision read 2026-08-27 (read 2026-08-27)
- Decoding ASTM, SAE and ISO grade markings for steel fasteners - American Fastener Technologies, edition not stated (read 2026-08-27)
- Understanding markings and grades on nuts, bolts and screws - Fasteners Plus, edition not stated (read 2026-08-27)
- Steel bolts - SAE grades - Engineering ToolBox, edition not stated (read 2026-08-27)
- Bolt grade chart - SAE, metric and ASTM - ConcreteCalculate, edition not stated (read 2026-08-27)