When doing automobile repairs or other machinery maintenance, we often need to install or replace the bolts and nuts. Each bolt is built to a specific standard regarding its tensile strength (the load that it would carry). People usually do not pay attention to the strength of the bolt when using them. For critical applications, proper strength class bolts need to be used. Both grades (in USA) and classes are used for strength classification. Here we would consider metric system (classes).
Below are few images of bold heads where the "strength classes" are marked with two number and a point in between. The first number when multiplied by 100 gives the minimum ultimate tensile stength (at which the bolt would break) in Mega Pascals (MPa) or equivalent units. The second number, when multiplied by 10, would give the percentage that the minimum yield strength is of the ultimate tensile strength (in MPa).
The first image shows "6.8", which means a minimum ultimate tensile strength of 600 MPa and a minimum yield strength of 600*0.8 = 480 MPa. Here the minimum yield strength is 80 percent of the ultimate tensile strength. This is a general construction bolt and preferably should not be used in critical load-bearing applications.
The bolt below shows marking of 10.9, which means minimum ultimate tensile strength of 1000 MPa and a minimum yield strength which is 90 percent of the ultimate tensile strength. It is a strong bolt and is actually used to tension alternator belt.
Below is an 8.8 class bolt, which should be the minimum class for automotive use.
Below is a very strong bolt of 11.9 strength class which means a minimum ultimate tensile strength of 1100 MPa. It is actually a cylinder head bolt, which obviously needs to be strong.
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