These pre-drilled bolts (sometimes called safety bolts) are meant to be used with a safety wire to prevent loosening through vibration or other forces. 

The wire maintains tension and remains in place by being twisted around itself and attached to the bolt on one end and an anchor point (which could be another bolt) on the other end. This method, also called Wire Locking, is a highly visible and easily inspectable indication that the fasteners are properly secured.

We machine these in-house and keep several sizes in-stock in both Hex Head and Socket Head. We can also custom drill safety holes to your specifications in a variety of materials and finishes.