It is much easier to remove them altogether as over time they no longer grip the little tag they are attached to pics below.
Also they rust and sometimes can not be undone, the screw head also gets damaged.
Remove them and replace them with a plastic block, the ones used for chipboard units etc, use a thin hacksaw blade and saw a line in the block, place it on the tag, then glue it in place where the metal clip used to be, drill 2.5/3mm hole into the block this will then take a stainless steel screw and hold great, no more rusting or trouble lining it up when you come to re-fit the bumper. Arctic
