For those that celebrate it, hope everyone had a wonderful holiday weekend. 

Back to the grind.  On to this week's mailbag:

Today's article will feature on the differences between CL 8.8/10.9/12.9 material.  What do those numbers mean?  

To start off, these classifications are strictly on metric parts.  The CL stands for "class" so the three designations we are going to examine in class 8.8, class 10.9 and class 12.9 metric fasteners. 

Class 8.8 fasteners (similar to grade 5) are a medium carbon quenched and tempered steel.  You will usually find these parts on any length below 16MM and up to 72MM.

Class 10.9 fasteners (similar to grade 8) are an alloy (a metal made by combining two or more metallic elements, especially to give greater strength or resistance to corrosion), quenched and tempered steel.  Class 10.9 fasteners are usually found on metric parts with a length ranging from 5MM-100MM.

Finally, there are the class 12.9 fasteners (similar to grade 9 and 1'' socket head cap screws).  This (like the class 10.9), are also composed of an alloy quenched and tempered steel.  The range on these parts are between 1.6MM-100MM.  

So what else sets these three classes apart.  Frankly it has to do with the proof load (maximum tensile force that can be applied to a bolt that will not result in plastic deformation), yield strength (the stress at which a specific amount of plastic deformation is produced) and tensile strength (the resistance of a material to breaking under tension).  

The higher the class (12.9 is the highest), the higher the proof load, lead and tensile strength.