Mortise locks are a unique kind of locking device which consists of one big rectangular body that is fitted into a mortise carved into the door frame. The entire locking machinery is found within the lock in the door itself. This includes instrumentation for operation of the deadbolt, latch and/or doorknob.

Like other locking mechanisms, mortise locks have standard keys, but they have been designed in such a way as to enable repeated rekeying. Rekeying is a method used to change the configuration of a lock in order to make old keys useless without having to change the entire locks. It is preferable since it is cheaper and ensures that you have full control over your locks, especially after moving to a new place.

Below are six steps to follow if you want to rekey your mortise lock. Remember that successful rekeying is about altering the cylinder of the mortise locking mechanism.

What you need

• Screw driver
• New mortise lock cylinder – you get this in two ways:
  • Take your current cylinder to the locksmith/hardware store and have it replaced or repined (changing tumblers which are the short pins inside the lock that fit with the key groves). This will ensure you get the right type. However, you won't be able to lock the house, so make appropriate arrangements.
  • Buy a DIY rekeying kit and change the mortise cylinder lock yourself. Ensure you buy one that is the same brand as your lock, or it will be useless. Tumblers are color-coded and there's an instruction sheet to show you how to rekey the cylinder.

Step 1: Inspect keyway positioning

With the door opened, use your screwdriver to take out the cover plate screwed onto the mortise cylinder. You will need to take out a screw at the top and bottom. Place them in a safe place since they will be used to fasten the plate on completion. Look at how the keyway is positioned within the cylinder; usually it's at the bottom. You can take a photo if you don't think you'll remember.

Step 2: Unfasten the cylinder

Use your screwdriver to unfasten the mortise lock cylinder from the rest of the locking system. If your mortise lock has more than one cylinder, you should expect to unscrew four screws before the cylinder can be pulled out.

3. Mortise lock cylinder

Take out the cylinder. Rotate your mortise lock anti-clockwise, and then take your new mortise lock cylinder and rotate it clockwise. This is very important; counter-threading your new cylinder may damage its entire mortise locking mechanism.

4. Lining up the cylinder

Ensure the mortise lock cylinders have been correctly lined up. Remember how the keyway was positioned and replace the new cylinder, ensuring that the keyway is positioned in exactly the same way. Fasten the new cylinder in place using its screws.

5. Testing

Before you finish, test the locking mechanism with your new keys to make sure the cylinder was properly replaced. If the lock doesn't work, rotate the cylinder anticlockwise for just one turn, taking care to maintain the keyway in the correct position. If this still doesn't work, you will need trim rings, which will help the cylinder to sit better. Contact a professional lockout service if you are unable to correct the problem yourself.

6. Restore cover plate

Having confirmed that your lock is working with the new key-set, screw the cover plate back on the side edge of the door. Before you close the door and try to lock it, confirm that the lock works with the door open. This is to ensure that your mortise lock rekeying was successful.