Nylock nuts have a nylon washer that’s pressed into the back of the nut. The washer is a bit smaller than the threads of the bolt, so when you thread it on there, it grabs em and keeps the bolt from shaking loose. This effect goes down every time you use reinstall the bolt, so limit it to a few install sessions. In practical terms, this is not an issue: Once you install a rack, you are not generally taking it on and off. We combine these Nylock nuts with beeswax on the threads of the bolt to insure everything stays tight. 18-8 stainless steel. M5 and M6 threads. These use a 8mm box wrench and a 10mm wrench respectively.