Mole grips?If it's not behaving consider replacing with a lashing.
I don't think those things are ever the same once they've been used and torn open.
You can close them with something like a G-clamp, small vice, waterpump pliers maybe?
Trouble is they suffer from UV degradation and break. That does not happen with S/S shackles.Use some sort of adjustable pliers, or also by hand pushing with one thumb over the other. I have them on my cockpit side clothes, they are >15yo and still going strong, opened/closed once or twice a year. Actually more difficult to open them, I insert the pliers long handle and twist them.
Trouble is they suffer from UV degradation and break. That does not happen with S/S shackles.
That’s not my experience with over a decade of use. They did break when I dumped the halyard and the speed of the sail dropping and stacking snapped a few. Now 5 years on from that, none have been damaged.