For standard fractional hardware it is Grade 5 and Grade 8, for metric it is Class 8.8 and 10.9. They are usually grouped Grade 5 and Class 8.8 as medium strength and Grade 8 and Class 10.9 as high strength. Choose the right fastener for the job. Just as diameter and length are important so are the specific properties of the fastener. Properties such as tensile, fatigue and shear strength must be taken into account.
Some applications call for the more flexible medium grade fasteners.
Medium grade fasteners can better withstand frequent disassembly and re-tightening. Remember, the reason for using a fastener is to secure two parts together. Choosing the wrong one will result in the squeaks, rattles and lost parts that you were trying to avoid in the first place. In order for a fastener to perform properly, it must be tightened properly. There are several methods used to measure a fastener s torque.