A door lock can be easily replaced by anyone who is reasonably familiar with woodworking tools.

Locks fall into one of four broad categories: mortise, cylindrical, tubular and rim or surface-mounted. The first three are set into the door so that only the knobs and latch bolts protrude; the forth is mounted on the surface of the door and is generally added on as an auxiliary to the main lock. s

Some rim locks are inexpensive spring-latch types that lock when the door is slammed shut, but the ones that offer the most protection are the deadbolt models that you lock by turning the key or a knob on the inside. Rim locks are mounted on the inside face of the door with a matching strike on the door jamb, and the better ones have a cylinder mechanism that goes through the door so they can be locked or unlocked from the outside with a key.

Mortise locks are recessed or set into the edge of the door, and they usually have a spring latch that locks automatically when the door is slammed shut, as well as a separate deadbolt that is activated by the key to "double lock" the door when leaving. (However, some inexpensive older models, especially those used on inside doors, may not have the extra deadbolt.) They are much trickier than tubular or cylindrical locks for the amateur toinstall because of the large and accurately sized mortise (recess) that must be carefully cut in the edge of the door.

Cylindrical locks have round housings and are much simpler to install -- all you have to do is bore a hole of the correct size through the door and another hole that intersects it through the edge of the door (for the latchbolt mechanism). Accurate positioning or locating of these holes is simple because of cardboard templates that are supplied by the manufacturer.

The better quality locks will have a combination spring latch and deadlatch, which prevents burglars from forcing them open by simple use of a plastic card or similar tool.

Cylindrical locks are usually designed with a keyhole in the outside knob and a locking button on the inside knob. For doors that have glass panels there are also cylindrical locks that have keyholes in both the inside and the outside knobs so a burglar cannot unlock the door by merely breaking the glass. Most of these locks are sturdily built for use on outside doors as well as on inside doors.

Tubular locks are similar to cylindrical locks except that they are much lighter in construction and are primarily designed for use on interior doors. Many of them do not have a deadlatching bolt behind the regular latchbolt the way cylindrical locks do, and many do not even have key locks -- just a button that you turn or push on one side and a small hole where a wire can be inserted to open the lock in an emergency on the other side.

When planning to replace an old lock with a new one the first step is removing the old lock so you can measure the size and spacing of the holes in the door. Start by first unscrewing the knob on the inside after loosening the small setscrew that locks it onto the spindle (you may have to hold the outside knob while you unscrew the inside one). Instead of a setscrew, or in addition to it, some knobs have a small button or catch on the side or bottom of the shank that must be depressed or pushed in before the knob will come free.

After the knob is off, the decorative rosette or plate can be snapped off (some are held on by screws). There may then be a second metal plate underneath that also must be unscrewed and slid off. Now the knob and lock assembly can be slid out from the other side of the door (the outside) after the screws that hold its rosette in place have been taken out.

The last step is removing the latch assembly. Take out the two screws that go through the plate at the top and bottom, then slide the whole latch assembly out from its hold or recess in the edge of the door.

If possible when selecting a new tubular or cylindrical lock, try to buy one that requires the same size holes in the door and one that has the same amount of "backset" (the distance from the edge of the door to the center of the knob spindle; it is usually a standard 2 3/4 inches but may vary in some models).

When a new lock requires a hole that is larger in diameter than the existing hole, you can almost always enlarge it to the right size by using a round rasp or round Surform tool. However, if your new lock is smaller in diameter than the old holes, then the problem is much more difficult -- it involves filling the old hole with a block of wood and then drilling a new one of the proper size. This is a tricky job even for a professional carpenter, so it should be avoided by looking for a lock that takes the same or a slightly larger diameter hole.

To remove an old mortise lock you take the handles off first, then, remove any decorative escutcheon plates on each side. Slide the spindle out that goes through the door, then loosen the screws that hold the lock onto the edge of the door and the entire lock assembly can then be slid out. In selecting a replacement for an old mortise lock, selecting one of the right size is even more important if you want to be able to fit the new mortise lock into the same opening -- and if you want the handle holes that go through the door to match.

In many cases when a mortise-type lock is to be removed, the owner wants to replace it with a modern cylindrical or key-in-a-knob type lock, but there will be some widely spaced holes in the door that the rosette on the new lock will not cover and there will also be that large mortise in the edge of the door that must be covered up so the new latch assembly can be properly mounted.

To solve these problems, and to eliminate the need for extensive carpentry that would involve filling in the old holes and then drilling new ones, manufacturers of door locks make special "modernization" kits. This kit contains oversize excutcheon plates that are long enough to cover the holes left by the old mortise lock, as well as provide a correctly positioned hole for the new cylindrical lock.

Also included in the kit is an oversize latch plate that is long enough to cover the old mortise opening in the edge of the door. It supports the latch assembly at the right height so it will line up with and engage with the lock mechanism. The only carpentry work called for is cutting a shallow recess for the latch plate in the edge of the door (above and below the mortise) so that the plate sits flush with the edge when installed.

Installing the new lock is basically the reverse of the procedures for taking the old lock out. Start by inserting the latch assembly first, then the knob and lock assembly from the outside, making certain the two engage properly. Then attach the knob and rosette assembly from the other side of the door.