The most common type of steel used for high-performance connecting rods is 4340 chromoly steel. 4340 has a tensile strength of 145,000 psi. Its hardness, ductility and other properties will vary based on the heat treatment applied to it. 4340 may also be referred to as aircraft grade or aircraft quality steel.
The entire steel manufacturing process determines the strength of these materials, as well. A simple designation of 4340 steel does not necessarily mean that two steel suppliers construct the final product to the same standards or with the same processes