The center weight on the HRX is exactly that, a weight. As such, no threading is provided. This only provides pressure on the record label and is intended to be used with the periphery ring clamp on VPI platters.
The Two piece delrin clamp of the lesser models utilizes a thin rubber fender washer on top of the platter. This washer works in combination with the dish on the underside of the two-piece clamp to force the record perimeter into contact with the platter since the periphery ring is not intended to be used.
In my experience with this clamping system, most people do not screw it down tight enough!. You will know when it is tight enough by tapping the inner lead-out groove with your fingernail to tell if contact with the platter has been made. If it is too tight, the record will begin to lift off the perimeter edge or your clamp will strip out! VPI does offer a stainless steel two-piece clamp as an option for encouraging snugger fitting.
Some vinyl records may not react well to the thickness of the rubber washer, this is analog after all
. Feel free to experiment with spindle washers from SME, Oracle or carbon fiber such as what is offered with the Shelter Harmony.
If your record if flat, give the whole clamping system a rest...