It is very simple to open the cat's eye. Open the door and use one hand to press the cat's eye outside the fixed door. The other hand can rotate the head counterclockwise to remove the cat's eye. You can directly measure the diameter of the cat's eyelet and the thickness of the door leaf, which is the easiest.
First buy your favorite cat's eye, then measure the inner diameter of the cat's eye, then buy the hole opener, pay attention to the electric drill when you open the hole, do not scratch the anti-theft door paint. In fact, there is no need to install it yourself, buy the cat's eye back, it is recommended to find a professional repair security door installation. A hole opener also needs a lot of money, you have to buy a hand drill, these tools are generally not used by the family, or it is more cost-effective to find a security door. When you open the hole yourself, measure the position of the front and rear panels. Otherwise, the front and rear openings will not be used against the security door.