Manufacturers of bolts and/or machines establish torque specifications for their components and the nuts and bolts that hold them together. Bolt torque, the amount of force required to tighten a bolt, is listed according either to the Society for Automotive Engineers (SAE), which uses U.S. measurements, or to metric units.
Metric bolt sizes are listed according to diameter, distance between threads and length. A bolt might be listed as 12 x 1.75 x 30. This indicates that the bolt has a diameter of 12mm, that the distance between each bolt thread is 1.75mm and that the bolt is 30mm long.
In the metric system, bolts are given grades according to its size, composition and design. The grades, or classes, include such common ones as Class 8.8 and Class 10.9.
Different bolt sizes and grades have different torque specifications. For example, 6mm bolts with a 10.9 grade have a maximum torque value of 10 foot-pounds, while the same-size bolts with a 12.9 grade have a maximum torque value of 12 foot-pounds. Consult your shop manual for the most precise listings.
To determine torque yourself, follow the formula T = K x U x D x P, where T is torque, K is 1.33, U is the coefficient of friction, D is diameter and P is preload.
Use 0.2 as the coefficient of friction for un-lubed bolts and 0.09 for lubed bolts.
To determine preload, take the published ultimate strength of the bolt, multiply it by the bolt's thread area and by 2/3.