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Torque Specifications for Bolts

Grade 8 bolt drawing

Most professional mechanics are aware of bolt grades, but many amateurs are not. When you replace the bolts on your vehicle, some are different than others in terms of strength. These strengths are called "grades" and are measured in tensile (pulling) strength. The harder you can pull on a bolt before it gives, the higher the grade likely is.

For example, a bolt with an SAE Grade 5 rating will have a higher tensile strength than a bolt with a standard grading (less than G5, usually called non-graded or Grade 2). This means it can withstand more tensile pressure than the lower grade bolt. Similarly, a Grade 8 bolt has a higher tensile strength than a Grade 5.

Head Markings

The grade of the bolt will be clearly marked on its head. SAE grades are marked with hashes (lines) in a wheel-like pattern. A 3-hash mark, which looks like the Mercedes logo, means Grade 5. A 6-hash mark, which looks like a wagon wheel, is an SAE Grade 8. Numbers stamped or raised on the bolt head are its metric grade, so 8.8 or 10.9 are the DIN Metric grades - the higher the number, the better the grade: 8.8 is roughly equivalent to G5 and 10.9 is roughly G8. An uncommon Grade 7 SAE marking has five hash marks.

What These Mean

The higher the grade, the greater the tensile strength due to a higher carbon content in the metal. This higher carbon comes at a price, however, and is manifested in the lower sheer strength, making the bolt more brittle to shearing (sideways cut).

Engineers balance these properties for the application at hand. So a bridge might be built entirely to eliminate sheer problems and rely entirely on Grade 8 bolts with high tensile strength. An automobile, however, has a lot of vibration, so some parts may be better suited to a Grade 5 bolt with better sheer strength or a non-graded bolt that will bend rather than break.

Using These Bolts

Anytime you remove a bolt from the vehicle and plan to replace it, you should be sure to be using not just the same size bolt, but the same grade as well. Engine head bolts, for instance, are nearly always specifically graded for the engine - often a Grade 8 or 10.9. Replacing these with a Grade 5 will mean that you will likely bend or break the bolt if you put it under the torque requirements for those head bolts.

To find the torque values for a given bolt by grade and size, see our chart here.

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Metric Bolt Torque Specs