To install HP rails in a threaded hole server rack, you need to unscrew the pegs in the mounting flange and use screws. The HP Rail Kits are simple and functional, exactly the way a Rail Kit should be. This is completely opposite from the older style HP Rail Kits that had complicated assemblies and mechanisms with lever arms and springs.
The HP Engineering Staff responsible for the design of HP Rail Kits definitely follows the “Don’t over Engineer it” philosophy. There are two pegs in the mounting flange of most HP Rail Kits. These pegs have a slight taper that allow it to install into a square hole server rack without the use of any tools. The Rail Kits lock in place with a very simple piece of spring steel the prevents the rail kits from moving.
If your server rack has round non-threaded holes like an IBM server rack, the pegs in the mounting flange will fit easily into the hole. If your server rack has a threaded hole then the HP Rail Kit is still compatible. The pegs must be removed by unscrewing them and then using screws to install the Rail Kit into the server rack.