If you look at the pic, it's slotted on and only turns clockwise. I tried to take it out (counterclockwise) and snapped the clip. Friction was enough that I thought it was still on. It is very possible that when you tightened it with a screwdriver, you also turned the outer portion enough that the clips lined up with the slot and eventually pressure pushed it out. Examine your old one, it might be fine. Mine was diffidently broken. The safety wire worked great to make sure the old one didn't fall out, but after getting the replacement, of course I did not use safety wire for it again. Although for a few days after that I kept checking to make sure I wasn't dripping any coolant there and also carried around a gallon of distilled water. But it hasn't leaked again and it's been several thousands of miles since.
