Watch Case Opener with 2 Flat Prongs For Removing or Replacing Screw On Watch Case Backs This simple but effective spanner has two blades that can be adjusted with the adjustment wheel in the middle of the tool handle, the blades will move from a diameter of 15mm up to 50mm. The blades fit into the rectangular slots on the back of the watch and you simply twist anti-clockwise to remove the back and clockwise to tighten it up.