Bolt Torque

Torque specifications frequent shop manuals. That’s a given. Today, you’ll sometimes find stretch measurements for certain fasteners along with torque angle figures but, generally speaking, “bolt torque” is still king. In many cases, the use of a torque wrench is the only way possible to measure the tension that is brought to bear on a fastener.

What is Torque?
Let’s rewind for a second. What is torque? Simple. Torque is the twist or the resistance to rotation. When used in reference to a fastener, torque is the resistance to turning of the bolt or nut. Torque is based upon the fundamental law of the lever: Force x distance = the torque or twist around a point. Torque is most commonly measured in foot-pounds or inch-pounds. In practice, if one pound of force is applied one foot from the center of the fastener, the resulting torque developed would be referred to as one foot-pound of torque. If the resistance measurement is in inches, the resulting torque would be referred to as twelve inch-pounds of torque.


Simple enough. Yet there’s more: Applying the exact amount of torque to a nut or bolt induces the correct amount of tension or elongation in the bolt that is necessary to hold the parts together. By applying the correct amount of specified torque to a bolt, the danger of distortion to the final part or adjoining parts is eliminated.

Stretch and Spring
It is also very important to understand that for a fastener to properly function, it must be “stretched.” ARP (the racing fastener company) notes that the ability of a fastener material to rebound (like a spring) is what really provides the clamping force. Different materials tend to react differently to these conditions. As a result, different fasteners are designed for different tasks.


What if you over-tighten a bolt? ARP points out that if a fastener is over-torqued, it stretches too much. Because of this, the yield will have been exceeded, and for all intents and purposes, the fastener is ruined.