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Torque Specs for Bolts

Bolt Head Markings

When doing automotive repairs or replacements, many mechanics will notice that some bolts look different than others. Although the protective coating (usually Zinc) may be a part of this, some bolts really are different from others in terms of grade (which refers to the metal type and subsequent strengths).

The torque specs for bolts are different according to the bolts measured tensile strength and its diameter and thread type/pattern. Bolts are classified by distinct markings on their heads which give their strength rating by one of two recognized national and international standards groups: the Society for Automotive Engineers (SAE) and the Deutsches Institut for Normung (DIN).

Bolt Specs

Bolt Head Spec Markings

A bolt with no markings on its head at all is a "standard" or SEA Grade 2 bolt. These have minimal tensile strengths and standards. Most bolts fall into this category.

Bolts with 3 hash marks from the corner of the hex are SAE 5, made of medium-carbon, tempered steel rated at 120,000psi tensile strength.

Bolts with 5 marks are SAE 7 and are uncommon, with a tensile strength of 150,000psi.

Bolts with 6 marks are SAE 8 (or "grade 8") and are very common in heavy applications with a tensile strength of 160,000psi.

Bolts with an 8.8 or 10.9 mark on their heads are DIN 8.8 and DIN 10.9 metric specifications. These are similar to SAE 5 and 8 respectively.

Using Spec'd Bolts

When replacing bolts on a vehicle that have a specification marking on their head, replacement bolts with the same specification should be used. A grade 8 might be "better" than a grade 5, but some attributes of the grade 8 (increased shear brittleness) may be unwise for the application.

Always replace like with like when it comes to graded bolts.

Bolt Torque Measurements

The torque (in pounds) you can put on a bolt when screwing it into a nut or threaded hole are dependent on several factors. The diameter of the bolt, its thread type, and the bolt's grade are all a part of this. The larger the bolt's diameter and heavier the thread, the more torque that can be applied. Whether or not the hole or bolt threads are oiled also plays a part.

Generally, if the bolt is meant to go into a slot and is of the right specified grade, then the manufacturer's recommended torque for that application are likely good.

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