Plug-in D-shackle brackets are designed to slide into a standard square hitch tube. This is a good way to add a shackle access point if you don’t have a sturdy bumper to work with. It’s best not to overtighten a shackle pin as it could seize in the threads when loaded. Tighten the pin until it gets snug and then back it out about half to one full turn. A sticking pin indicates the shackle has probably been overloaded and distorted.