D.I.D. Standard chain sized 520 with 120 links D.I.D. Standard Roller 520 cahin with 120 links. D.I.D. is the #1 chain manufacturer in the world. Solid rollers. Shot-peened parts. High cylindrical and roundness accuracy. Exacting tolerances provide low friction. Pre-stretched. Extended riveted bushing "Anti-Kink" design. Special alloy steel. Average tensile strength of 7,200 lbs.