The bi-directional bolt allows clockwise and counterclockwise nuts to spin the opposite direction to tighten and loosen.

IMPORTANT, the bi-directional nut does not work well with the bi-directional bolt and catches often.

If you want to print the double ended bolt, make sure to print the nuts first and then pause the print to put the nuts on.

Original model at thangs.com: https://social.thangs.com/m/808209