There are two basic types of lock for the main doors of a house:
A simple night latch lock does not provide adequate security on its own. They are too easily forced to provide adequate security on their own.
A better option is the deadlock type cylinder rim lock. These are designed to lock as the door is closed but can also be double-locked with the key. This means that the lock cannot be operated by turning the knob on the inside which will prevent easy exit should an intruder break in elsewhere.
Marking out for the lock
The lock will be supplied with detailed measurements for fitting, usually in the form of a template. This is held against the door in the required position and the various fixings are marked.
The lock has a protruding edge which should be recessed into the door. Mark this recess using the template or by holding the lock against the door.
Drilling and cutting
Use a sharp chisel to cut the outline and pare out the waste. Check the lock fits neatly in the recess and its lip sits flush with the door edge.
Mark the centre of the hole for the cylinder itself and bore this out using a drill bit of the specified diameter. Drill from the inside to a point where the bit just begins to emerge through the face of the door. Stop drilling and work from the outside thus preventing the exterior wood from splitting.
Be careful to hold the drill at right angles and level so that the hole runs true.
Insert the cylinder through its mounting plate ( like a large washer ) and into the door. Depending on the thickness of the door, you may need to trim the bar which operates the locking mechanism. Check how far this bar needs to sit into the lock, and trim accordingly. Hold the bar in a vice on your workbench and cut with a hacksaw. Once trimmed, use a file to clean off the burr to ensure easy fitting and smooth operation.
Fitting the lock
Fasten the backplate to the door and fix the cylinder using the screws provided. Mount the lock itself onto the backplate and secure this with screws provided.
Hold the strike plate over the latch, close the door as far as the frame edge and draw round it with a pencil line. To ensure that there will not be too much 'free play' in the door, the strike plate must be positioned to hold the door firmly against the door stop. See door latch strike plate fitting. The plate must be recessed into the frame as specified to provide adequate security. Notch out the recess using a sharp chisel. Fix loosely with one screw, and check alignment before fitting the remaining screws and tightening.