[GTO-relic]

This begs the question, whats the difference between SD421 4140 steel rods, and 58-62 370/389 1140 steel rods.

They are very close. There's some chromium, molybdenum, silicon added to the SD421 4140 rod. But they are within a whisker of each other.

SAE-AISI 1140 Steel vs. SAE-AISI 4140 Steel :: https://share.google/vGhyqtoh9sXnOeU4A

[Shiny]

Originally Posted by GTO-relic

This begs the question, whats the difference between SD421 4140 steel rods, and 58-62 370/389 1140 steel rods.

They are very close. There's some chromium, molybdenum, silicon added to the SD421 4140 rod. But they are within a whisker of each other.

SAE-AISI 1140 Steel vs. SAE-AISI 4140 Steel :: https://share.google/vGhyqtoh9sXnOeU4A

Generic answer you know: same carbon content so similar properties are possible. 4140 will be a LOT easier to harden and will be tougher. And you also know you will pay for that hardenability.

Another big difference is likely sulfur content. I think 1140 is typically re-sulferized to make it machine easy, but I'm no expert. Those sulfide inclusions are great for making chips on a mill but not so great for fatigue or even toughness.

Another possible difference may be in cleanliness. Just guessing, but I'd expect the cost of 4140 to also deliver more QA and less crap in the metal. This won't matter much for overall strength but could make a HUGE difference in fatigue performance.

Net: 1140 costs less to make and machine, may be good enough if part is never stressed above fatigue limit and the manufacturing process can consistently deliver enough strength through the part section. Pontiac and that vertically integrated world made it happen. IMO, 4140 would be a safer bet today to get the properties you want.