Why is it so hard to make a sending unit seal up?

I'm on the 3rd sending unit now. So far, not one of them has sealed up properly. I have a brand new gas tank. Give me some instructions on how to install a sending unit correctly, because evidently I'm too retarded to do it or something because I can't get them to seal up. I'm so sick of wasting $5 here and $5 there on gas that just leaks on the floor.

Nevermind, I got it. Turns out, it musta been the damn tabs holding it out only a tiny bit. Thought I was gonna go nuts there for awhile.