A deadbolt is one of those jobs that looks like a twenty-minute swap until the old one will not come out and the new one will not line up with the frame. Your father may tell you the lock still works. It probably does. But if the key is worn, the bolt sticks, or the door has been rattling for years, it is time to replace it, and it is worth doing in the right order.

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Before you buy anything

Take the old lock off the door first and look at it. Measure the backset, which is the distance from the edge of the door to the center of the bolt hole. Measure the diameter of the hole in the face of the door. Measure the thickness of the door. Note which side the thumb turn is on, if there is one. The package will list the sizes it fits. Buy the one that matches what you measured, not what the old lock looked like in the store.

Gather the tools

You will need a drill with the right bit, a screwdriver, a pencil, a tape measure, and a small level. Keep the old screws in a cup so none of them go missing. Lay the new parts out on a towel so you can see the order they go in.

Remove the old lock

Work from the inside of the house. Remove the screws that hold the inside half in place, then pull the outside half free. Pull out the strike plate on the frame and look at the mortise in the door frame. If it is splintered or loose, stop and decide whether it needs repair before the new lock goes in. A new deadbolt in a loose frame will not hold.

Install the new lock

Feed the bolt assembly through the door, make sure the latch and cylinder are seated, and attach the outside and inside halves together. Tighten the screws evenly. Do not overtighten them, because stripped screw holes are harder to fix than a lock that is snug.

Set the strike and test it

Close the door slowly and watch where the bolt meets the strike plate. If the bolt hits the frame instead of sliding into the hole, adjust the strike plate. Then test the lock from both sides. Turn the key fully in both directions. Make sure the bolt throws all the way out and all the way back. Open and close the door several times. Then test the lock again with the door open, to make sure the mechanism is smooth and not binding.

Cut the spare keys

Make at least two spare keys before you hand over the new one. Label none of them. Put one somewhere in the house he will remember, and give one to a neighbor or a family member you trust. Do not hide a spare under a mat or in a flowerpot by the door, where anyone can find it.

When to ask someone else

If the door is metal, has a fire rating, or the frame is cracked or rotten, bring in a locksmith or a carpenter. The same is true if the lock is part of an entry system you do not understand. Ask any professional to show you how to test the lock before they leave, and to tell you which key goes with which door.

Tonight

Check that the bolt slides fully into the frame and that the key works from outside. If it does not, leave the lock as it is, and call someone in the morning. A door that almost locks is worse than one you know is only old.