Originally Posted by Rob Roy's Revenge

There is a thin tube on the inside of the filler neck, sometimes the tube can be bent outwards towards the inner wall of the filler neck and it can foul the inside of the filler caps wall. If that is the case, bend the tube slightly inwards to avoid it from fouling the inner wall of the filler cap.

That happened to my Iron 883's filler neck, I bent the tube in slightly and I haven't had a problem installing the filler cap since.

Originally Posted by HOTLAP

I got one of these years ago because mine rattled, works great, looks cool and it’s tamper proof.