A bolt grade or property class identifies specified mechanical properties; it does not set the working load of the whole joint. Common SAE J429 hex bolts use three radial lines for Grade 5 and six for Grade 8. Metric bolts commonly show numbers such as 8.8, 10.9 or 12.9.

When replacing a bolt, check the grade, thread and finish together. A bolt that fits the hole can still be unsuitable for the joint. Head markings help identify the strength system; color and head shape do not.

For the distinction between fastener families, see our bolt vs screw guide. This article focuses on strength requirements and identification.

How to Identify Common SAE Hex Bolt Head Markings

For these common SAE hex-head patterns, the number of radial grade lines plus two gives the grade. Read the manufacturer’s identifier separately. If corrosion or damage obscures the marking, verify the grade through traceable documentation rather than guessing.

Grade, Property Class and Material Are Different Specifications

SAE J429 applies to specified inch-series steel fasteners. ISO 898-1 applies to specified metric carbon-steel and alloy-steel bolts, screws and studs. Grade or property class, material, coating and product geometry are separate parts of a complete fastener specification.

SAE Bolt Grade Chart: Grades 2, 5 and 8

Values below are in ksi (1 ksi = 1,000 psi). This summary covers selected SAE J429 grades and diameter ranges.

*For Grade 2 diameters from 1/4 through 3/4 in, the higher values apply to bolts and screws 6 in long or shorter, and to studs of all lengths. Longer bolts and screws use the lower Grade 1 mechanical requirements: 33 ksi proof stress, 36 ksi minimum yield strength and 60 ksi minimum tensile strength.

Reference: SAE J429_201405, Table 1, selected mechanical requirements. The diameter and Grade 2 length conditions above are part of the summary; other grades, tests and requirements are omitted.

Grades 5 and 8 specify heat-treated fasteners. Their material and processing requirements come from SAE J429; a steel alloy name alone does not demonstrate grade compliance.

LILY lists Grade 5 steel hex head screws and Grade 8 steel hex head screws separately. Compare the required specification as well as diameter and length.

Metric Bolt Strength Chart: 8.8, 10.9 and 12.9

Values below are in MPa (1 MPa = 1 N/mm²). ISO 898-1:2013 covers coarse threads M1.6–M39 and specified fine-pitch combinations from M8 × 1 to M39 × 3. For the classes shown, Rp0.2 is the stress at 0.2% permanent elongation. It is different from the stress used in the proof-load test.

Reference: ISO 898-1:2013, Table 3, selected properties. Here d is the nominal thread diameter. The 8.8 requirements change above 16 mm; specified diameter/pitch combinations and product loadability also apply.

Some low-head and countersunk fasteners have reduced loadability because their head geometry limits strength. The material’s property class alone does not establish the tensile capacity of those products.

What the two numbers mean

For the designation 8.8, 8 × 100 gives a nominal tensile strength of 800 MPa. The second 8 indicates a nominal yield-to-tensile ratio of 0.8, giving 640 MPa. This designation rule explains the label; the actual acceptance requirements come from the standard’s tables.

For 10.9, the designation gives nominal values of 1,000 MPa and 900 MPa. The tabulated minimum tensile and 0.2% proof-strength values shown above are 1,040 MPa and 940 MPa. Do not replace those tabulated minima with the simpler nominal calculation.

For metric procurement, compare Class 8.8 hex head screws and Class 10.9 hex head screws against the drawing’s full thread and product specification.

Grade 5 vs Grade 8: Compare Strength Within the Same System

For diameters from 1/4 through 1 in, Grade 8 has a minimum tensile strength of 150 ksi, compared with 120 ksi for Grade 5—a 25% increase. This compares the specified tensile minima, not the allowable working loads of assembled joints.

A stronger replacement does not automatically improve the joint. The nut, tapped hole, washer or clamped material may govern performance. Fatigue, shear, bending and corrosion need separate assessment, and the tightening procedure must suit the complete assembly.

Is SAE Grade 8 the Same as Metric Class 8.8?

No. SAE Grade 8 and metric Class 8.8 use different designation systems. Grade 8 has a 150 ksi minimum tensile strength, approximately 1,034 MPa. Class 8.8 has an 800 MPa minimum through 16 mm, or 830 MPa above 16 mm within the range shown.

Grade 5 and Class 8.8 are broadly similar strength categories; Grade 8 and Class 10.9 also have close tensile minima. These comparisons are useful for orientation. They are not substitution specifications: inch and metric threads and assembly requirements still differ.

Proof, Yield and Tensile Strength: Which Number Matters?

  • Proof stress (Sp) is the stress used to calculate the specified proof load. In the proof-load test, the fastener must meet the standard’s permanent-elongation acceptance criterion.

  • Yield strength and 0.2% proof strength (Rp0.2) describe specified deformation limits. Rp0.2 is the stress corresponding to 0.2% permanent elongation; it is not the same quantity as Sp.

  • Tensile strength (Rm) is the maximum tensile load in the applicable test divided by the relevant original cross-sectional area. For the threaded-load example below, use the nominal tensile stress area.

For an M10 × 1.5 Class 8.8 fastener capable of carrying the full specified tensile load, the nominal tensile stress area is 58.0 mm². Using the 800 MPa minimum tensile strength gives 58.0 × 800 = 46,400 N, or 46.4 kN. This is a minimum tensile-load calculation, not an allowable service load or a tightening-torque recommendation.

Stainless Steel and Structural Bolts Need Their Own Identification

A2-70 and A4-80 are stainless fastener designations under the ISO 3506 system, not ISO 898-1 property classes. Likewise, ASTM structural bolts use their specified grade markings and installation requirements. Do not identify them by borrowing the SAE radial-line rule.

For stainless or structural applications, order to the required specification. A similar tensile strength does not demonstrate equivalent corrosion resistance, toughness or assembly compatibility.

A Practical Replacement and Purchasing Check

  • Record the full thread designation, length, head style and required product standard.

  • Specify the grade or property class separately from material and coating.

  • Match the required nut and washer specifications; do not assume bolt and nut markings use identical rules.

  • Check temperature, corrosion and reuse requirements in the equipment documentation.

  • Request the required compliance documents and lot identification when traceability is part of the purchase specification.

For related fastener categories, browse our fastening and joining products.

ISO 898-1:2013 evaluates mechanical and physical properties at test temperatures from 10 °C to 35 °C. Those values are not unrestricted operating-temperature ratings. For the scope and exclusions, see the official ISO 898-1:2013 summary.

To check a replacement, contact us with the drawing, clear photographs of the head markings, and the required grade, thread and finish.

Frequently Asked Questions

Can different bolt grades be mixed in one assembly?

Follow the assembly’s fastener schedule. Different grades may be specified for different positions, but do not substitute or mix grades at equivalent positions without approval from the responsible engineer. Matching diameters alone does not establish compatibility with the tightening procedure.

Can a hardness test alone establish a bolt grade?

No. Hardness ranges can overlap between grades, and hardness does not verify all required properties. Confirm compliance using the tests and documentation required by the governing specification.

Does a certificate need to show the exact alloy chemistry?

It depends on the purchase order and governing specification. A certificate of conformity states compliance; a material or inspection test report can provide specified test results. Request chemistry results and lot traceability explicitly when they are required.