4. Two-way lock nut (Center lock nut)
There is a small mark in the middle position on the outside of the nut, almost like a punch right in the center, that shows it’s a two-way nut, as the name suggests, this kind of nut is locking in two ways, whether you put it on or off, once you start to screw up to that size, it will start to lock. Both Stover lock nut and two-way lock nuts use distorted threads to keep the nuts from loosening. The difference is that the distorted thread of Stover nuts is at the top and two-way nuts have it located in the center.