Three radial lines on a bolt head mean SAE Grade 5 (120 ksi / 827 MPa minimum tensile up to 1 in), six lines mean Grade 8 (150 ksi / 1034 MPa), and no lines means Grade 2 or unmarked (74 ksi up to 3/4 in). Metric bolts carry the ISO 898-1 class number instead: 8.8 = 800 MPa tensile (830 MPa above 16 mm), 10.9 = 1040 MPa, 12.9 = 1220 MPa; stainless bolts carry the ISO 3506 grade, such as A2-70 (700 MPa). The charts below give minimum proof, yield and tensile strength for each mark, from Portland Bolt's and Bossard's published copies of SAE J429, ASTM and ISO 898-1 / ISO 3506 (the standards themselves are paywalled). The head drawings are simplified; "XYZ" stands for the manufacturer's mark every graded bolt must also carry. To turn a grade into a tightening torque use the Bolt Torque & Preload Calculator or the Bolt Torque Specs chart.
SAE and ASTM inch bolt head markings
SAE J429 Grade 1
No grade mark. Low or medium carbon steel.
1/4 - 1-1/2 in: proof 33, yield 36, tensile 60 ksi
SAE J429 Grade 2
No grade mark. Low or medium carbon steel.
1/4 - 3/4 in: proof 55, yield 57, tensile 74 ksi
7/8 - 1-1/2 in: proof 33, yield 36, tensile 60 ksi
SAE J429 Grade 5
3 radial lines, 120° apart. Medium carbon steel, quenched and tempered.
1/4 - 1 in: proof 85, yield 92, tensile 120 ksi
1-1/8 - 1-1/2 in: proof 74, yield 81, tensile 105 ksi
SAE J429 Grade 8
6 radial lines, 60° apart. Medium carbon alloy steel, quenched and tempered.
1/4 - 1-1/2 in: proof 120, yield 130, tensile 150 ksi
ASTM A307 Grade A
307A + maker's mark. Low or medium carbon steel.
1/4 - 4 in: tensile 60 ksi
ASTM A449 Type 1
A449 + maker's mark. Medium carbon steel, quenched and tempered.
1/4 - 1 in: proof 85, yield 92, tensile 120 ksi
ASTM F3125 Grade A325 Type 1
A325 + maker's mark (Type 3 weathering steel: A325 underlined). Medium carbon steel, quenched and tempered.
1/2 - 1-1/2 in: proof 85, yield 92, tensile 120 ksi
ASTM F3125 Grade A490 Type 1
A490 + maker's mark (Type 3: A490 underlined). Medium carbon alloy steel, quenched and tempered.
1/2 - 1-1/2 in: proof 120, yield 130, tensile 150-173 ksi
ASTM A354 Grade BD
BD + maker's mark (may also carry 6 radial lines). Medium carbon alloy steel, quenched and tempered.
1/4 - 4 in: yield 130, tensile 150-173 ksi
ASTM A193 Grade B7
B7 + maker's mark. Medium carbon alloy steel, quenched and tempered.
1/4 - 2-1/2 in: yield 105, tensile 125 ksi
ASTM A193 Grade B8 Class 1
B8 + maker's mark. Type 304 stainless steel.
all in: yield 30, tensile 75 ksi
ASTM A193 Grade B8M Class 1
B8M + maker's mark. Type 316 stainless steel.
all in: yield 30, tensile 75 ksi
ASTM F3125 A325 Type 3
A325 underlined + maker's mark: weathering (atmospheric-corrosion-resistant) steel.
Same strength as Type 1: proof 85, yield 92, tensile 120 ksi.
ASTM F3125 A490 Type 3
A490 underlined + maker's mark: weathering steel.
Proof 120, yield 130, tensile 150-173 ksi.
SAE J429 and ASTM strength by grade
| Grade / specification | Diameter (in) | Proof min. ksi (MPa) | Yield min. ksi (MPa) | Tensile min. ksi (MPa) |
|---|---|---|---|---|
| SAE J429 Grade 1 | 1/4 - 1-1/2 | 33 (228) | 36 (248) | 60 (414) |
| SAE J429 Grade 2 | 1/4 - 3/4 | 55 (379) | 57 (393) | 74 (510) |
| SAE J429 Grade 2 | 7/8 - 1-1/2 | 33 (228) | 36 (248) | 60 (414) |
| SAE J429 Grade 5 | 1/4 - 1 | 85 (586) | 92 (634) | 120 (827) |
| SAE J429 Grade 5 | 1-1/8 - 1-1/2 | 74 (510) | 81 (558) | 105 (724) |
| SAE J429 Grade 8 | 1/4 - 1-1/2 | 120 (827) | 130 (896) | 150 (1034) |
| ASTM A307 Grade A | 1/4 - 4 | — | — | 60 (414) |
| ASTM A449 Type 1 | 1/4 - 1 | 85 (586) | 92 (634) | 120 (827) |
| ASTM F3125 Grade A325 Type 1 | 1/2 - 1-1/2 | 85 (586) | 92 (634) | 120 (827) |
| ASTM F3125 Grade A490 Type 1 | 1/2 - 1-1/2 | 120 (827) | 130 (896) | 150-173 (1034-1193) |
| ASTM A354 Grade BD | 1/4 - 4 | — | 130 (896) | 150-173 (1034-1193) |
| ASTM A193 Grade B7 | 1/4 - 2-1/2 | — | 105 (724) | 125 (862) |
| ASTM A193 Grade B8 Class 1 | all | — | 30 (207) | 75 (517) |
| ASTM A193 Grade B8M Class 1 | all | — | 30 (207) | 75 (517) |
Reading the inch markings
- SAE J429 marking rule (quoted by Portland Bolt): screws and bolts of 1/4 in and larger must be marked on top of the head, raised or depressed, with the grade symbol and the manufacturer's mark; hex heads may be marked on the side. Studs and screws smaller than 1/4 in need not be marked. SAE J429 grades stop at 1-1/2 in.
- Grade 2 strength drops above 3/4 in (to the Grade 1 values), and the 1/4-3/4 in values apply only to headed bolts up to 6 in long; longer ones fall back to Grade 1.
- ASTM A325 and A490 were withdrawn in 2016 and replaced by ASTM F3125 grades A325 and A490; the head still reads A325 or A490. Type 3 (weathering steel) is shown by underlining.
- A354 BD must be marked "BD" and may also carry the six Grade 8 lines (Portland Bolt). A "-" in the table means the specification has no requirement for that property.
- Sources disagree on Grade 8 material: Portland Bolt describes it as medium carbon alloy steel; Bossard's SAE J429 table describes it as low carbon martensitic steel. Both give the same 120 / 130 / 150 ksi proof, yield and tensile minimums.
Metric (ISO 898-1) and stainless (ISO 3506) head markings
ISO 898-1 class 4.6
Class number + maker's mark (marking compulsory only from 5.6 on studs; on hex screws from 4.6 at d ≥ 5 mm).
Tensile 400 MPa, yield 240 MPa, proof 225 MPa.
ISO 898-1 class 8.8
"8.8" + maker's mark; the dot may be left out.
d ≤ 16 mm: tensile 800, yield 640, proof 580 MPa.
d > 16 mm: tensile 830, yield 660, proof 600 MPa.
ISO 898-1 class 10.9
"10.9" + maker's mark.
Tensile 1040, yield 940, proof 830 MPa.
ISO 898-1 class 12.9
"12.9" + maker's mark (common on socket head cap screws).
Tensile 1220, yield 1100, proof 970 MPa.
Reduced loadability "0" prefix
A leading 0 (08.8, 010.9) marks a screw whose head or shank cannot carry the full class load (ISO 898-1 revision of April 2009).
Same material as 8.8, but the product standard limits its load, so reduce the tightening torque.
ISO 3506 stainless A2-70
Steel group + property class + maker's mark, required on hex and socket screws from M5.
Austenitic (304 type), tensile 700, 0.2% proof 450 MPa.
ISO 3506 stainless A4-80
As above; A4 is the molybdenum (316 type) grade.
Tensile 800, 0.2% proof 600 MPa.
ISO 898-1 property classes: minimum strength
| Property class | Diameter (mm) | Tensile Rm min. (MPa) | Yield min. (MPa) | Proof stress Sp (MPa) | Elongation A min. (%) | Hardness (HV) |
|---|---|---|---|---|---|---|
| 4.6 | all | 400 | 240 (ReL) | 225 | 22 | 120-220 |
| 4.8 | all | 420 | — | 320 | — | 130-220 |
| 5.6 | all | 500 | 300 (ReL) | 280 | 20 | 155-220 |
| 5.8 | all | 520 | — | 380 | — | 160-220 |
| 6.8 | all | 600 | — | 440 | — | 190-250 |
| 8.8 | d ≤ 16 mm | 800 | 640 (Rp0.2) | 580 | 12 | 250-320 / HRC 22-32 |
| 8.8 | d > 16 mm | 830 | 660 (Rp0.2) | 600 | 12 | 255-335 / HRC 23-34 |
| 9.8 | d ≤ 16 mm | 900 | 720 (Rp0.2) | 650 | 10 | 290-360 / HRC 28-37 |
| 10.9 | all | 1040 | 940 (Rp0.2) | 830 | 9 | 320-380 / HRC 32-39 |
| 12.9 | all | 1220 | 1100 (Rp0.2) | 970 | 8 | 385-435 / HRC 39-44 |
Reading the metric markings
- What the number means: in ISO 898-1 the nominal tensile strength of class 10.9 is 1000 MPa and its nominal yield 900 MPa (class 8.8: 800 and 640 MPa), so the first number is a hundredth of the nominal tensile strength and the decimal is the yield-to-tensile ratio. The specified minimums in the table are slightly higher than the nominal values.
- Where marking is compulsory (Bossard, per ISO 898-1): manufacturer's mark and property class on hexagon screws of classes 4.6 to 12.9 and on hexagon or hexalobular socket head cap screws of 8.8 to 12.9, for thread diameters of 5 mm and up, preferably on the head. Studs of 5 mm and up are marked from class 5.6 up, preferably by an indentation on the threaded part.
- Class 8.8 changes at 16 mm: tensile 800 → 830 MPa, yield 640 → 660 MPa, proof 580 → 600 MPa.
- 4.8, 5.8 and 6.8 have no specified yield for machined test pieces; ISO 898-1 gives full-size values (340, 420 and 480 MPa) that it says are "under investigation", so they are left out of the table.
- Nuts (ISO 898-2) are marked with a single class number such as 8 or 10 on the bearing face or side from 5 mm thread size, or with a clock-face code.
- Rough ASTM equivalents (Portland Bolt): 4.6 ≈ A307 Grade A; 8.8 ≈ A449 or F3125 A325 up to M24; 10.9 ≈ A490 or A354 BD; 12.9 ≈ A574 socket head cap screws only. Portland Bolt stresses these are rough and that an engineer should approve substitutions.
ISO 3506 austenitic stainless property classes
| Property class | Steel grades | Tensile Rm min. (MPa) | 0.2% proof Rp0.2 min. (MPa) | Elongation after fracture (mm) |
|---|---|---|---|---|
| 50 | A1, A2, A3, A4, A5 (not A8) | 500 | 210 | 0.6 d |
| 70 | A1-A5, A8 | 700 | 450 | 0.4 d |
| 80 | A1-A5, A8 | 800 | 600 | 0.3 d |
| 100 | A4, A5, A8 only | 1000 | 800 | 0.2 d |
ASTM F593 inch stainless bolts
| Head mark | Alloy group | Condition | Diameter (in) | Tensile (ksi) | Yield min. (ksi) |
|---|---|---|---|---|---|
| F593A | 1 (304 family: 303, 304, 304L, 305, 384, XM1, 18-9LW, 302HQ, 303Se) | AF | 1/4 - 1-1/2 | 65-85 | 20 |
| F593B | 1 | A (annealed) | 1/4 - 1-1/2 | 75-100 | 30 |
| F593C | 1 | CW1 (cold worked) | 1/4 - 5/8 | 100-150 | 65 |
| F593D | 1 | CW2 (cold worked) | 3/4 - 1-1/2 | 85-140 | 45 |
| F593E | 2 (316, 316L) | AF | 1/4 - 1-1/2 | 65-85 | 20 |
| F593F | 2 | A (annealed) | 1/4 - 1-1/2 | 75-100 | 30 |
| F593G | 2 | CW1 (cold worked) | 1/4 - 5/8 | 100-150 | 65 |
| F593H | 2 | CW2 (cold worked) | 3/4 - 1-1/2 | 85-140 | 45 |
Reading the stainless markings
- ISO 3506: A = austenitic; A2 is the 304 type, A4 adds molybdenum (316 type). The class number is one tenth of the minimum tensile strength. Hexagon and socket head screws from M5 must show steel group, property class and maker's mark. Bossard's caution: stainless parts not marked to the standard "will often only correspond to property classes A2-50 or A4-50".
- Commercial A2 and A4 screws are class 70 (Bossard). Elongation for stainless is measured on the whole screw and given as a length, e.g. 0.4 d for class 70.
- ASTM F593 (inch): the head shows "F593" plus a letter for alloy group and condition (A to D for group 1, the 304 family; E to H for group 2, 316) and the maker's mark (Portland Bolt). Cold-worked CW1 applies only up to 5/8 in.
- Stainless bolts are much weaker than alloy steel grades: Portland Bolt notes that A193 B8 Class 2, the strongest stainless grade it knows, is "not close to the strength of alloy grade 8 bolts".
Sources
- Portland Bolt & Manufacturing Co., Fastener Identification Markings, retrieved 2026-10-06 (www.portlandbolt.com/technical/fastener-identification-markings/). Used for: SAE J429 and ASTM grade marks, material, proof, yield and tensile minimums.
- Portland Bolt & Manufacturing Co., ASTM F593 (specification summary), 2020 (www.portlandbolt.com/technical/specifications/astm-f593/). Used for: inch stainless F593 alloy groups, conditions, strengths and marks.
- Portland Bolt & Manufacturing Co., Metric to Imperial Grade Equivalents (FAQ), 2014 (www.portlandbolt.com/technical/faqs/imperial-astm-equivalents-for-metric-grades/). Used for: approximate ASTM equivalents of metric classes.
- Bossard, Screws property class 4.6 to 12.9 (Fastening technical information, pp. 12-16), 01/2025 (assets.eu.ctfassets.net/0vp0u5uh75zd/1VPwGxSRcAQdDRArMtTqeY/8fe01cf6ef692e134b45f895fb69fe97/012_016_Screws_property_class46_Fastening_EN_01_2025.pdf). Used for: ISO 898-1 tensile, yield, proof stress, elongation and hardness.
- Bossard, Screws, bolts, nuts: marking (Fastening technical information, pp. 21-24), 01/2025 (assets.eu.ctfassets.net/0vp0u5uh75zd/51POoWyCl9DHZL24SSRUW/40b10f8c735f4db4e42d613597c76204/021_024_Screws_bolts_nuts_Fastening_EN_01_2025.pdf). Used for: SAE J429 table with MPa, ISO 898-1 marking rules and reduced-loadability marks.
- Bossard, Stainless steel fasteners (Fastening technical information, pp. 29-38), 01/2025 (assets.eu.ctfassets.net/0vp0u5uh75zd/4x0XlZ4jmVDayHkCwm6bJa/9e75bd94ce96f715cfaf56a06860ebc5/029_038_Stainless_steel_fasteners_Fastening_EN_01_2025.pdf). Used for: ISO 3506 steel groups, property classes 50/70/80/100, marking and corrosion guidance.
- Portland Bolt & Manufacturing Co., SAE Grade Markings (FAQ), 2015 (portlandbolt.com/technical/faqs/sae-grade-markings/). Used for: the SAE J429 marking rule.
- The standards themselves (SAE J429, ASTM F3125, A307, A449, A354, A193, F593, ISO 898-1, ISO 898-2, ISO 3506-1) are sold by their publishers; values here are the manufacturers' published reproductions named above. MPa for inch grades are converted (1 ksi = 6.8948 MPa) and rounded.
Frequently asked questions
What do the lines on a bolt head mean?
They give the SAE J429 grade. Three radial lines mean Grade 5 (medium carbon steel, quenched and tempered: 120 ksi minimum tensile, 92 ksi yield up to 1 in). Six radial lines mean Grade 8 (150 ksi tensile, 130 ksi yield). A head with no grade lines is Grade 1 or 2, or unmarked, and should be treated as low strength (Grade 2: 74 ksi tensile up to 3/4 in).
Is a class 8.8 bolt the same as Grade 5?
Close but not identical. Class 8.8 has a minimum tensile strength of 800 MPa up to 16 mm and 830 MPa above, with 640 or 660 MPa yield; Grade 5 has 120 ksi (827 MPa) tensile and 92 ksi (634 MPa) yield up to 1 in. Portland Bolt lists A449 or F3125 A325 as the rough ASTM equivalent of 8.8 and says an engineer should approve any substitution.
What does 10.9 on a bolt mean?
It is the ISO 898-1 property class. The first number is one hundredth of the nominal tensile strength (10 → 1000 MPa) and the decimal is the ratio of yield to tensile (.9 → 900 MPa nominal yield). The specified minimums are a little higher: 1040 MPa tensile, 940 MPa yield and 830 MPa proof stress. 10.9 is roughly equivalent to Grade 8 or A490.
What does A2-70 mean on a stainless bolt?
A2 is the steel group in ISO 3506: austenitic chromium-nickel stainless (the 304 type, 18-8). 70 is one tenth of the minimum tensile strength, 700 MPa; its 0.2% proof strength is at least 450 MPa. A4-80 is the molybdenum-bearing 316-type grade at 800 MPa. Bossard warns that stainless screws not marked to the standard often only meet class 50 (500 MPa).
What is the difference between proof load, yield and tensile strength?
Proof load is the tension a finished bolt must carry with no permanent stretch; yield strength is where it starts to stretch permanently; tensile strength is where it breaks. For Grade 5 up to 1 in they are 85, 92 and 120 ksi. Torque charts usually aim the clamp load at about 75% of proof load (Portland Bolt) to stay clear of yield.