Blown347Hatch: The difference between 4340 and 4130 is huge. While both are vacuum produced "Chromolly" steels, but 4340 has about twice the moly of the 4130. In the annealed state, they are about the same in tensile, but once they are heat treated, the game changes. Both of them must be oil quenched after they come out of the furnace. Importantly, how well a part quenches is a function of how thick it is. A thick part will be harder at the surface than in the middle where it cools more slowly. For example, 4130 typically is Rockwell "C" 35 at up to 5/16 deep and only Rockwell "C" 28 at 1/2".

So, what's the difference? 4130 is easier to forge and easier to machine, so it is no suprise that many forged rods are 4130. The big difference is in the tensile strength if the parts are heat treated. After tempering, 4130 will reach a tensile of about 125,000 PSI, but 4340 will reach 270,000 PSI - well over double the strength of 4130. Either one will probably do the job in the Lightning. Without heat treating, both are about 75,000 to 85,000 PSI as forged.