Torque bolts are sized in either SAE (Society of Automotive Engineers) imperial or metric measurements. Each bolt has a number of identifiable dimensions. The most obvious is length, but a bolt is also identified by its diameter and thread spacing. In addition, bolts are graded by the iron they are made of, with stronger metals able to withstand more torque before failing.
As mentioned above, bolts are sized by three physical dimensions: major diameter, thread spacing, and length. Major diameter measures the distance from the peak of the thread on one side of the bolt to the same on the far side. Spacing is just that: the distance between threads on the bolt. Length is the length of the thing.
Metric bolts record the major diameter in the form of "M#," where the hash is replaced by the measurement in millimetres. Thread spacing is measured in millimetres between the threads. Length is also measured in millimetres.
This information is coded in a standard way: D - S x L, where D is the major diameter, S is the thread spacing, and L is the length of the bolt. A bolt two centimetres long with a major diameter of eight millimetres and thread spacing of 1.25 millimetres is designated "M8 - 1.25 x 20." A metric bolt four centimetres long with spacing of one millimetre and a major diameter of one centimetre would be coded "M10 - 1.00 x 40."
Like SAE bolts, metric bolts have the strength of the metal stamped on the cap. Unlike SAE bolts, this information is recorded as numbers in the form of "X.Y" although that period is not a decimal point.
The first number, X, is one per cent of the bolt's nominal tensile strength in Newtons per square millimetre (1 N/mm2 equals 145 psi). The second number, Y, is ten times the ratio of nominal yield stress to nominal tensile strength (X). A class 8.8 metric bolt's nominal tensile strength would be 8 x 100 = 800 N/mm2. Its nominal yield stress would be its tensile strength divided by Y, in this case 800 x 0.8 = 640 N/mm2.
Common classes of metric bolts are 5.8, 8.8, and 10.9. These numbers are roughly equivalent to SAE grades 2, 5, and 8, respectively. You can also find metric bolts of class 12.9.
SAE bolts record their grades with radial lines on the cap. Add two to the number of lines to determine the grade. No lines indicated grade 2, three lines indicates grade 5, and six lines means grade 8.
In both SAE and metric bolts, higher numbers mean stronger bolts.
Dry and Lubed Torque
Torque is measured as foot-pounds in imperial units and as Newton-meters (N-m) in metric. One foot-pound equals 1.356 N-m. Small values of torque will sometimes be measured in inch-pounds--naturally, there are 12 inch-pounds per foot-pound.
The maximum torque a bolt should withstand depends on its diameter, on its strength, and on whether or not the bolt has been lubricated with machine oil. A dry M10 bolt of class 10.9 is rated for a maximum torque of 75 N-m. The same bolt lubed is rated for only 70 N-m. As a rule of thumb, lubrication with standard machine oil reduces the maximum torque by 10 per cent.
Consult an expert to determine the maximum torque for any given bolt. You can also try one of the tables linked to in the References section here.
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