I use a torx 6 on my f3 sapphire and f5s sapphire band pins.
I just got my Fenix 5 and was also looking to put on an existing leather strap (normal type, not quickfit). How do you undo the torx screws as the bar just seems to rotate - do you need two torx screwdrivers, one for each side simultaneously to grip the other side? Or am I missing something? Thanks...