Panning clamp question

Hi all, I have just received a Sunwayfoto panning clamp to use on my Arca Swiss Z1 ballhead, but the included M6 screw (as well as the screw for the ballhead) is too short. There is a gap after which the threads start inside the ballhead stem, and the screw threads don't reach deep enough.
If you are using panning clamps from RRS or some other company, can you tell me how long the screws are approximately? Does anyone actually use a panning clamp with the Z1. Any help will be greatly appreciated.