Hex Head Bolt MarkingsThe strength and type of steel used in a bolt is supposed to be indicated by a raised mark on the head of the bolt. The type of mark depends on the standard to which the bolt was manufactured. Most often, bolts used in machinery are made to SAE standard J429, and bolts used in structures are made to various ASTM standards. The tables below gives the head markings and some of the most commonly-needed information concerning the bolts. For further information, see the appropriate standard. Often one will find "extra" marks on a bolt head--marks in addition to those shown below. Usually these marks indicate the bolt's manufacturer. ASTM A325 Type 2 bolts have been discontinued, but are included below because they can be found in existing structures. Their properties can be important in failure investigations. While the bolts shown below are among the most common in the U.S., the list is far from exhaustive. In addition to the other bolts covered by the SAE and ASTM standards, there are a host of international standards, of which ISO is perhaps the most well known. Information provided from Unified Engineering, Inc. |
| Bolt Grade Markings and Strength | ||||||
| Head Markings | Grade or Class | Material | Nominal Size Range ( Inches ) |
Mechanical Properties | ||
| Proof Load ( psi ) | Minimum Yield Strength ( psi ) | Minimum Tensile Strength ( psi ) | ||||
| American | ||||||
 No Markings |
Grade 1 & 2 | Low or Medium Carbon Steel | 1/4 thru 3/4 |
55,000 | 57,000 | 74,000 |
| 3/4 thru 1-1/2 |
33,000 | 36,000 | 60,000 | |||
 3 Equal Spaced Radial Lines |
Grade 5 | Medium Carbon Steel, Quenched and Tempered | 1/4 thru 1 |
85,000 | 92,000 | 120,000 |
| 1 thru 1-1/2 |
74,000 | 81,000 | 105,000 | |||
 3 Lines |
Grade 5.2 | Low Carbon Martensite Steel, Quenched and Tempered | 1/4 thru 1 |
85,000 | 92,000 | 120,000 |
 5 Radial Lines |
Grade 7 | Medium Carbon Alloy Steel,, Quenched and Tempered | 1/4 thru 1-1/2 |
133,000 | ||
 6 Radial Lines |
Grade 8 | Medium Carbon Alloy Steel, Quenched and Tempered | 1/4 thru 1-1/2 |
120,000 | 130,000 | 150,000 |
 6 Lines |
Grade 8.2 | Low Carbon Martensite Steel, Quenched and Tempered | 1/4 thru 1 |
150,000 | ||
| Stainless markings vary. Most stainless is non-magnetic | 18-8 Stainless | Steel alloy with 17-19% Chromium and 8-13% Nickel | 1/4 thru 5/8 |
80,000 to 90,000 |
100,000 to 125,000 |
|
| 3/4 thru 1 |
45,000 to 70,000 |
100,000 | ||||
| Above 1 | 80,000 to 90,000 |
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| ASTM Bolt Designations | ||||||
 |
A307 | Low Carbon Steel | 1/4 thru 4 |
60,000 | ||
 3 Radial Lines |
A325 Type 1 | Medium Carbon Steel, Quenched and Tempered | 1/2 thru 1 |
85,000 | 92,000 | 120,000 |
| 1-1/8 thru 1-1/2 |
74,000 | 81,000 | 105,000 | |||
 3 Lines |
A325 Type 2 | Low Carbon Martensite Steel, Quenched and Tempered | 1/2 thru 1 |
85,000 | 92,000 | 120,000 |
| 1-1/8 thru 1-1/2 |
74,000 | 81,000 | 105,000 | |||
 Underlined |
A325 Type 3 | Weathering Steel, Quenched and Tempered | 1/2 thru 1 |
120,000 | ||
| 1-1/8 thru 1-1/2 |
105,000 | |||||
 |
A490 Type 1 | Alloy Steel, Quenched and Tempered | 1/4 thru 1-1/2 |
150,000 | ||
 Underlined |
A490 Type 3 | Weathering Steel, Quenched and Tempered | 1/4 thru 1-1/2 |
150,000 | ||
| Metric | ||||||
 8.8 |
Class 8.8 | Medium Carbon Steel, Quenched and Tempered | All Sizes thru 1-1/2 | 85,000 | 92,000 | 120,000 |
 10.9 |
Class 10.9 | Alloy Steel, Quenched and Tempered | All Sizes thru 1-1/2 | 120,000 | 130,000 | 150,000 |
| Stainless markings vary. Most stainless is non-magnetic | A-2 Stainless | Steel alloy with 17-19% Chromium and 8-13% Nickel | 1/4 thru 5/8 | 80,000 to 90,000 |
100,000 to 125,000 |
|
| 3/4 thru 1 | 45,000 to 70,000 |
100,000 | ||||
| Above 1 | 80,000 to 90,000 |
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| Tensile Strength: The maximum load in tension ( pulling apart ) which a material can withstand before breaking or fracturing. Yield Strength: The maximum load at which a material exhibits a specific permanent deformation Proof Load: An axial tensile load which the product must withstand without evidence of any permanent set. |
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