My serpentine belt needed changed out, as we changed it we also took the top pulley off to give better leverage for getting the belt on. While driving the idler pulley fell off and upon inspection of the pulley, I noticed the bearing in the inside was shot. We replaced it, and took it for a drive. Within 5 minutes the idler pulley at fallen off at was last to the world. I'm ready for round 3 but why is the idler not staying on! We watched and when we turned the car on the nut slightly moved, we tightened each time but it was still loose! How do we get the idler pulley on there in the CORRECT assembly and how does it stay?

 

What Fit do you have?