Yes, it might well be if you compared the steel grade. It is not solely the metallurgy of the rods. If you get new H beams, you get 7/16" bolts. Original fasteners were nuts & bolts, bolts which were smaller in diameter & reduced clamp force where you want maximum strength. Cutting the factory rod for the bolt head did two things that further weakened the rod: introduce a sharp edge & removed material in a vital spot. Then you have the precision CNC machining of new rods on modern equipment, compared to a shop closing & honing old factory rods by hand. If the old rods have pressed pins, new pins will never be as tight as the originals.
Is that enough reasons?
