Chicago Bolt. These are a uniquely-shaped type of bolt. Unlike other bolts, which have threading along the outside of a cylindrical shaft, the Chicago bolt has a hollow shaft that is smooth on the outside and threaded along its interior. This form is also known as a "female" thread. Chicago bolts fit together with regular ("male") bolts in the same way that normal nuts do. However, as they have a head and a shaft component, they are still classified as bolts.