It takes three locks, placed at least a foot apart, to keep a burglar from levering your door open. One is probably part of the knob or latch. You can add deadlocks, rim locks, or police locks in any combination. For best results, install one above and one below the latch. Always use different keys for each lock.
Types of locks
A deadlock is a solid bolt embedded in the door. The bolt shoots into a keeper embedded in the jam.
A rim lock is secured to the inside face of the door so that it overhangs the edge and meets with a keeper on the frame. The most secure have interlocking latches.
A police lock is also secured to the door’s inside face. The commonest are the double-throw type, in which two bars slide into keepers on the frame, and the vertical prop type, in which a steel rod reaches to the floor.
The latch on the door must align with its keeper on the frame. Install the door
side first; then position the keeper. When you buy a lock, tell the locksmith whether your door opens in or out and which side is hinged. The lock should come with a template showing what size holes to cut and where to position them. The package should also contain hardware, although it is often better to use longer screws than those provided. For more security, drill holes through the door and secure the casing with carriage bolts, putting the nuts inside. Cover the heads with a hardened steel plate, available from most locksmiths.
To install a deadlock, cut two holes in the door: a large one for the lock cylinder and a smaller one for the bolt. After positioning the centers of both holes with the template, use an expansive drill bit or a hole saw to cut the cylinder hole.
Drill the smaller hole in the edge of the door so that it meets the cylinder hole; use a drill guide to keep the bit level and square. (If you don’t have a guide, ask a helper to watch at eye level while you watch from above.) You may have to shape the small hole with
a chisel to accommodate the bolt assembly and mortise the faceplate into the door’s edge.
To mark the position of the keeper on the jamb, dab a little lipstick or crayon on the end of the bolt, close the door, and turn the knob. Then drill a hole in the jamb large enough for the bolt. Center the strike plate over it and
trace the plate’s outline. Chisel a mortise so that the plate sits flush with the jamb; screw the plate in place.
To install a rim lock or a police lock, only the lock-cylinder hole must be cut. Position the keeper of a rim lock by closing the door and marking where the latch hits. If the door is inset into the frame, cut a mortise for the keeper. Use a chisel to cut a little at a time until the door closes properly.
To position the keepers of a doublethrow police lock, turn the knob until the bars are fully extended; then place the keepers over them and mark their positions. For the prop type, put the top of the steel rod in the casing, with the lock closed, and mark the place where the bottom hits the floor. Then cut a mortise for the plate that holds the rod and screw the plate into place.