Mortise locks were the standard locks on older doors. In most remodelling projects where old doors are retained, modernisation dictates that mortise locks be converted to modern cylindrical locks. A mortise lock rests in a large cavity that's carved--or mortised--from the edge of the door. To complete the conversion this cavity must be filled with wood before the new lock can be installed.
Remove the set screw from the inside knob. You'll find it under the edge of the knob running into the square spindle resting in the centre of the knob. Turn the screw counterclockwise with a screwdriver to remove it. Slip the knob off of the spindle, then grasp the outside knob and pull to remove the entire spindle.
Locate and remove the four screws in the face plate on each side of the door using a screwdriver. These are located in the corner of each plate. Pry the plates from the door with the tip of a sharp wood chisel.
Remove the two screws holding the plate in the edge of the door. Pry the lock from the door using a small, flat screwdriver.
Measure the mortise hole, from top to bottom and side to side. Insert your tape measure into the mortise until the tip hits the back, then measure the depth of the hole.
Cut a piece of pine lumber to fill the hole. Cut the piece to the proper thickness and width on a table saw, then cut it to length using a mitre saw. Insert the block into the hole to test the fit. It should fill the hole snugly without being forced. Trim it to fit as needed if it's too tight.
Apply wood glue to the faces of the block that will be inside the door. Insert the block into the door and nail it in place with two 1-inch finish nails. Fill the remaining space around the block and the holes in the door face with wood filler.
Allow the filler and glue to dry completely, then sand the door smooth with a power sander and 100 grit sandpaper.
Apply the paper template of your new lock to the door as directed in the instructions that come with the lock. In general, it will fold around the edge of the door and provide a placement diagram for the cylinder hole in the face of the door and the striker bolt hole in the door's edge. Mark them both by tapping a nail through the paper in the indicated position.
Bore the cylinder hole with a 2 1/8-inch hole saw on a variable speed drill. Apply steady pressure and keep the drill at full speed. Do not force it through the other side, to prevent splintering.
Bore the striker hole in the edge of the door with a 1-inch paddle bit. Drill through into the cylinder hole you bored in the door's face, keeping the drill level.
Center the striker bolt collar on the 1-inch hole in the door's edge. Mark the outline. Cut the material from the outline with a wood chisel and mallet to a depth of 1/8 inch. Remove the old striker plate from the jamb and repeat the outlining and chiselling process for the new striker plate, centred on the hole in the jamb. Install the striker plate on the jamb with the screws provided.
Insert the striker bolt through the hole in the edge of the door. Attach it using the two screws provided. Insert the two halves of the new knob lock through the hole in the striker bolt assembly. Check that the knob with the thumb toggle is on the inside of the door, and the keyhole is to the outside. Align the screw holes, thread the screws through and tighten with a screwdriver.