1. To prepare the mortice slot for the lock, start by marking the centre of the thickness of the door. The body of the lock can be used as a template which you should hold against the correct position on the door and mark with a pencil, draw around it carefully always keeping it central to the mark that you made.
Use a drill and chisel to remove the wasted wood. When drilling, always use a bit which matches the thickness of the lock back case, not the lock face.
Square the edges of the mortice slot so that the lock back case mechanism fits snugly. Mortice Dead and Sash Locks in 65mm and 75mm sizes and with Np (Nickel Plated) or Pb (Polished Brass) faces are available on our website. Choose wisely.
2. Hold the body of the lock against the face of the door and use it as a template, mark the position of the keyhole and the hole for the spindle of the handle.
Drill through the thickness of the door in both of these positions, avoid splintering by holding a block of wood on the opposite side of the door from which you are drilling and in the same position you are drilling, this will allow the drill bit to pass safely into the block of wood.
3. Cut out the shape of the keyhole using a padsaw or small drill. Use an open or covered escutcheon (keyhole cover) to give a tidy finish to the keyhole.
4. Once you have fitted and screwed the lock into the slot you have made, check that it is working properly.
5. Close the door until it is almost completely shut and then mark on the jamb (door frame) the position where the striking plate (keeper) will be fixed, once you have determined this, chisel out the recess in the door frame for the striking plate (keeper) and screw into position."
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Info supplied by www.DirectDoors.com