Mortise locks are the standard of a bygone era. Today's cartridge cylinder knobs are more versatile and secure, not to mention easier to install. When modernizing a lock from a mortise to a modern knob, you will need to fill the void left by the lock mechanism before boring for the new lock. The best way is to fill the space with a wooden block, sized to fit snugly, then proceed as if working with a new slab door. You will also have additional spaces to fill around the edges. Auto body filler hardens better than traditional wood filler and will not shrink.
Removing the Old Lock
Locate and remove the screws in the plates on both sides of the door. There are typically four, one near each corner.
Remove the screw under the edge of the inside knob to release it from the spindle and pull both knobs from the door.
Remove the two screws holding the lock into the door from the door's edge. Pry the lock gently from its hole with a small flat screwdriver.
Remove the two screws holding the striker plate to the jamb and remove the striker plate.
Filling The Mortise
Measure the slot, or mortise, in the edge of the door, front to back and top to bottom. Measure the depth of the mortise by inserting the tape measure into the door, until it hits the back of the mortise. Mortise locks vary widely in size. The dimensions of the hole will be somewhat larger than the lock you removed.
Use a table saw to cut a block of pine to the proper thickness and dimensions to fill the void. It is best to cut it to fit easily, with the edge that will be flush with the door's edge 1/16 inch long. This can be planed or sanded smooth.
Insert the wood block into the mortise to check for fit. Remove the block, spread wood glue on all faces that will be inside the door, and reinsert. Nail the block in place with a 1-inch finish nail near the edge of the door, close to the top of the block.
Fill the holes from the lock spindle and screws of the old lock with auto body filler, using a flexible putty knife. Allow the glue and putty to set overnight. Sand the auto body filler smooth and plane the block at the edge of the door, using a grater-style plane and sanding for a smooth finish.
Installing the New Lock
Apply the paper template from your new door knob to the door. Cut the hole in the face of the door, centered on the mark, with a 2-inch hole saw. Use steady pressure and keep the drill at full speed. Do not force the drill out the other side, as this will cause splintering.
Mark the center of the striker bolt hole on the edge of the door and bore the hole with a 1-inch paddle bit, from the edge of the door, into the larger hole in the face of the door.
Mark the jamb centered between the inside edge of the jamb and the door stop molding, centered on the new door knob hole, height-wise. Bore a 1-inch hole, 5/8-inch deep in the jamb.
Position the striker bolt collar on the edge of the door, centered on the 1-inch hole you bored. Mark around it with a pencil. Cut the material from the door's edge with a chisel inside the outline, to a depth of 1/8 inch. Repeat with the 1-inch hole in the jamb, using the striker plate as a template.
Insert the striker into the door's edge. Drive one short screw, provided with the hardware, into the door through each hole in the striker bolt collar. Attach the striker plate to the jamb in the same way. Insert the two halves of the new knob through the door, aligning the spindle with the hole in the tail piece of the outside knob. Thread the two long knob screws through the holes and tighten them with a screwdriver.