Once you’ve got access to the third floor of the police station you’re well on your way in Resident Evil 2. At that point you’re likely working on trying to leave the Police Station by getting the Garage Key. One of the objectives in doing that is going to the Clocktower to find an electrical part that will help you open a jail cell where the key is on a corpse.
To complete this puzzle you’re going to need the Large Gear. This is located in the third floor storage area in the East Storage Room. To get to this area you will need to at least have gotten the crank and you’ll probably be much farther than that at this point.
The large gear can be found in the third floor storage area in the East Storage Room. Basically in the center of this room that filled with shelves (and a couple zombies). The large gear is a two-slot item so make sure you have enough space in your inventory to hold the item.
Once you’ve got the Large Gear you can take it across to the Clocktower if you’ve opened up the shortcut door between the East Storage Room and the Main Hallway. You will need to place the Gear into the machine to lower the stairs and then put the gear into another machine at the top of the stairs to ring the bell and retrieve one of the electronic pieces that can open the jail cell that you need to open to retrieve the Parking gate code.