Door locks: tougher than ever.

There are basically two kinds of burglars: skilled professionals, often with tools, who check out a neighborhood before deciding which homes to target; and opportunistic thieves, who may simply kick doors until they find one that gives way. The right lock can deter the pro and defeat the opportunist.

Nearly two-thirds of all burglaries involve forced entry, and they succeed partly because of flimsy door locks and mounting hardware. But even poorly performing locks, we found in our tests of 31 models, can be significantly strengthened by replacing the strike (the metal plate that's mounted on the door jamb and into which the lock's bolt slides) with a $10 box strike. We used the Mag Security Box Strike, sold at Home Depot. You may not have to buy a new lock at all; a box strike should add significant protection to your present lock.

Another inexpensive way to enhance your current door lock is to replace short mounting screws with 3-inch ones. Screws of that length will penetrate the thin door jamb to reach the studs, providing more resistance to impact.

This group of locks represented an improvement over the ones we tested seven years ago. Then, just one Medeco lock provided excellent resistance to kicking, using its included hardware. This time, 10 locks scored excellent in that test, using the strikes with which they came. Adding a stronger strike improved the kick-in resistance of all the weakest locks; three actually went from the lowest to the highest score in that test.

Other highlights of our door-lock tests:

* Only the expensive high-security locks were able to stand up to an assault with a standard cordless drill (as might be used by an amateur crook) and by our consulting locksmith's efforts at picking.

* Of the three keypad electronic locks we tested, only one model provided adequate kicking resistance.

HOW TO CHOOSE

Decide how much lock you need. See First Things First, below, for the pluses and minuses of each type of lock. Most can be installed by a homeowner, unless the size or type is different from what's on the door now. In that case, you may need to bring in a locksmith.

Decide how much you can spend. For the ultimate in protection, plan on spending more. High-security locks were the only ones that could withstand picking and drilling. The top-scoring locks were two Medeco models and one from Assa; all cost between $160 and $200.

Consider your door. Double-cylinder deadbolts provide better protection than single-cylinder models for doors with glass in or close enough to them that a burglar could break the glass and reach in. With a double-cylinder deadbolt you need a key to open the lock from inside the house as well as from the outside. If you install one, make sure all family members know where the key is, in case they have to get out fast in an emergency.

Check out the strike and screws. Whatever type of door lock you buy, be sure your strike is made of heavy-duty metal and installed with 3-inch screws. If the lock doesn't come with a box strike, you can buy one at most hardware and home-improvement stores. While you're there, pick up some 3-inch screws.

Think about weathering. Even if no would-be burglar ever puts the strength of your door and door lock to the test, the weather can take a toll on the lock's polished brass finish over the years. Most models did well in our accelerated-weathering tests, which subject locks to a salt-water spray, but the Falcon D841 door lock corroded badly.

First things first Consider your security needs and the type of lock that is already on your door.

Any of these types of locks provides more security than a key-in-knob entry set, the kind found on most homes' doors.

SINGLE-CYLINDER DEADBOLT

Outside, a metal collar surrounds the lock cylinder. Inside, there's a thumb-turn handle.

Pluses Many models from which …