You are standing in the hallway at two in the morning because the front door key has started sticking, and you want it fixed before your dad has to wrestle with it in the dark. Breathe. A lock is one of the few jobs in the house that goes in a clear order, and this chapter walks it in that order: keys first, then the choice, then the install, then the test.
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Start with a count of the keys
Your mom probably had a ring of keys, and some of them may now belong to people who no longer come by. Before you touch the door, sit down at the kitchen table and gather every key you can find for the front, back, and garage entries. Write down who has each one: your dad, you, a neighbor, a cousin who used to house-sit, the spare hidden under a planter. Those answers decide whether a simple rekey is enough or whether the whole cylinder should be replaced. If you cannot account for a key, that is the strongest reason to change the lock.
Rekey or replace
Rekeying keeps the same lock body and gives it a new set of pins, so the old keys stop working and one new set takes their place. It is usually the quieter, less involved path for a door that still turns well, and a locksmith can often do it on site. Replacing the deadbolt means new hardware, and it makes more sense when the bolt drags, the door has been forced, the finish is worn through, or the lock is an older style you cannot key the way you want. When you call, ask three things: will every key be changed in one set, will you give me a written count of the new keys, and will you take the old keys away with you. Ask too whether the new lock can be keyed alike with the other doors so one key opens them all. That question saves a drawer full of keys later.
Check the door before you buy anything
If you are replacing, measure before the store trip. Look at the edge of the door where the lock sits. Measure the thickness of the door, then measure the backset, which is the distance from the edge of the door to the center of the hole where the handle goes. Most residential deadbolts use a backset of either 2 3/8 inches or 2 3/4 inches, and a mismatched backset is one of the most common reasons a new lock comes back to the store. Note the bore diameter, which is the hole the cylinder slides into. Take the old lock out and bring it with you, or photograph the back of it. Check the package for the lock's grade and whether it is rated for an exterior door. Those printed ratings are the things to compare side by side, not a clerk's word.
Install it in order
Work with the door open and the bolt pulled back. Lay the old screws on a towel so the small ones do not vanish into the carpet. Mark the strike plate position by closing the door and reading exactly where the bolt meets the frame. Drill the frame hole straight, then set the strike plate. Use the long screws that reach past the trim into the solid wood of the frame, because the short screws in most boxes will not hold a bolt against a hard push. Install the cylinder and the inside trim, slide the bolt, and tighten the screws in a cross pattern so nothing binds. If the door rubs or the bolt catches, do not force it. Loosen the strike and move it a little at a time until the bolt slides clean.
Test it until it is boring
A lock is finished when nothing about it surprises anyone. Close the door, turn the key, and watch the bolt throw all the way out and pull all the way back. Turn the thumb turn from inside. Open and close the door five times. Lock it from the outside, unlock it with the key, and try each key in the set. Lean gently on the door and check that the latch and the deadbolt both hold it without any shake. Then hand one key to your dad, keep one for yourself, and put the spare somewhere you both know by heart, not under the planter.
When to stop and call someone
If the key will not turn, the bolt does not throw fully, the door will not close after the change, or the frame has split, stop and call a locksmith or a carpenter. Ask them to write down what they changed and what they left alone. Your dad may want to stand in the hallway while the work is done, and that is fine. Let him watch. If you need a locksmith close to home, the St. Louis list on Local Help for a Widowed Father in St. Louis is a place to start.
Good-week card: He reached for the key on its hook without having to ask where it was.