Torque Spec

There is often confusion about what "torque spec" really means. Most mechanics with any experience understand that some bolts require a specific torque in some applications. For instance, the head bolts on a car engine may have a torque requirement of 85lbs.

But what does that actually mean, other than you need to set your torque wrench to that number of pounds and then tighten accordingly?

Fasteners, Grades, Torque Values

Various threaded fasteners (i.e. nuts and bolts) are used to hold different components in place on your vehicle's engine, body, frame, etc. As these bolts and nuts are tightened with a wrench, they stretch ever so slightly. The more torque you put on them, the more they're likely to stretch.

The amount of stretch will depend on the fastener type, size, thread count, and the total torque being applied. Fasteners come in "grades" which denote the amount of carbon in their metallic makeup. Each grade, in a specific diameter and thread count, has a different range of torque allowances before being compromised.

For example, on our Metric Bolt Torque Specs page, we show the various bolt diameters and grades along with the maximum torque allowed in a chart.

We also showed you how to find the grade of the bolt you're working with in Torque Specifications for Bolts.

Holding Force

The primary purpose of a fastener is, of course, to hold things together. Generally, the larger the diameter of the bolt, the wider the thread spacing (space between each thread or measurement from peak-to-peak or valley-to-valley).

A relatively small bolt of 8mm diameter will have a wider thread spacing than a smaller bolt of say 6mm. That difference become crucial when tightening and holding. The wider the space between threads, the more the bolt is tightened with each turn.

That leads to the 6mm bolt having a much smaller "holding" ability than does the 8mm. An 8mm bolt can have a 3,000 pounds (14,000 Newton) force when tightened properly. The 6mm is less than 2/3 of that figure.

Torque Specs

On the most critical components of any assembly - be it your vehicle's engine, a bicycle pedal, etc. - the manufacturer will offer torque specifications for the fasteners involved.

Those will usually be given in psi or pounds per square inch. Specific tools are required to measure the amount of force being applied when tightening fasteners to a specific torque. If the torque is misapplied, it can lead to the fastener coming loose, to components breaking (from too much force being applied), and worse.

Always apply proper torque using the right techniques when working with materials and components that require it.

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Metric Bolt Torque

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Axle Nut Torque Specs

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