Does anyone know how to tighten a swivel clamp base? Mine slides when using a calibrator thus when stringing it loses a lot of tension
Tighten the two nuts underneath the clamp base (your first pictures). You want them to sit a few mm away from the bush that blocks the clamp base to the rail when the handle is on the open position so that the clamp base can move freely. When you turn the handle to lock the clamp base the nuts will come up and press against the bush. If you find that you have to readjust the nuts to often some threadlocker on the bolt.
I have same problem. I think maybe do some cleaning of the base area to get rid of any oil will help.