You will often find mortise locks in older houses. Sometimes you have to replace the mortise lock cylinder when you are in the process of restoring your home. Fortunately, the process is easier than you think. All it takes is a little time and patience.
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Open the door on which you wish to change a mortise lock cylinder. You'll find the cover plate that is over the mortise lock cylinder. Remove the two screws that keep the cover plate in place, and put them in a safe place. Check for the keyway's position in the cylinder. On many mortise lock cylinders, the keyway is located at the bottom of the cylinder.
Take off the cover plate, and take out the screw that keeps the mortise lock cylinder in place. On some mortise lock cylinders, there may be more than one screw, especially if there is an inner and outer cylinder.
Remove the cylinder from the mortise lock by turning it counter-clockwise. Set it aside. Insert a new cylinder by turning it clockwise. Make sure you do not counter thread the new cylinder.
Make sure the mortise lock cylinder lines up properly so that the keyway is at the bottom, just as it was for the original mortise lock cylinder you replaced. Insert the set screw or screws and tighten them.
Test the lock to see if it works properly. If it doesn't, try turning the mortise lock cylinder one turn counter-clockwise, making sure the keyway is still at the bottom. If it still doesn't work, try getting trim rings from your local hardware store that will help the mortise lock cylinder you replaced fit better.
Put the cover plate back on the door so it covers the mortise lock cylinder, and put the screws back in.