Residential Locks and Rekeying
Pin tumbler theory, rekeying by hand, deadbolt grades and the door hardware failures that actually let people in.
Pin Tumbler Theory
The pin tumbler is the lock you will open, rekey and sell more than any other. Almost every house in North America runs on it. Understand the shear line and you understand rekeying, picking, master keying and most of what goes wrong in a worn cylinder.
- The shear line is the boundary between plug and housing. A key works by splitting every pin stack there.
- Binding on insertion points to spacing. Smooth insertion with no turn points to depth.
- A lock that works only with the worn original has worn to fit that key.
- Dry lubricant only. Oil collects dust and creates a paste inside the cylinder.
The shear line
A pin tumbler cylinder has a rotating plug inside a fixed housing. Drilled through both are five or six chambers. Each chamber holds a spring, a driver pin above and a key pin below. With no key in the lock the springs push the drivers down so they cross the gap between plug and housing, and the plug cannot turn.
The correct key lifts each key pin exactly to the point where the top of the key pin meets the bottom of the driver pin at the boundary between plug and housing. That boundary is the shear line. With every stack split at that line there is nothing crossing the gap and the plug rotates freely.
Everything else follows from this. A key cut too shallow leaves a driver still crossing the line. A key cut too deep pushes a key pin across it instead. Both feel identical to the customer, which is a dead key, and both are diagnosed by looking at cut depth rather than by replacing the lock.
Depths, spacing and why a key sticks
A key is described by two things. Spacing is the distance from the shoulder to each cut position, and depth is how deep each cut is. Manufacturers publish both as a depth and space chart. Kwikset uses seven depths on a 0.023 inch increment. Schlage uses ten depths on a 0.015 inch increment, which is why Schlage keys look more finely stepped.
A key that goes in but binds is usually a spacing error, because the pins are not sitting at the bottoms of the cuts. A key that goes in smoothly and will not turn is usually a depth error. Learning to separate those two symptoms saves an enormous amount of guesswork.
The ramps between cuts matter too. If adjacent cuts differ by too many depth increments the ramp becomes a cliff, the key pin cannot climb it, and the key jams. Every manufacturer publishes a maximum adjacent cut specification for exactly this reason and a code cutting machine respects it automatically.
Wear, and the lock that only works one way
Cylinders wear predictably. Brass key pins mushroom at the tip, springs weaken, and the plug face gets a lip worn into it by years of key insertion. The classic symptom is a lock that works with the original key and not with a fresh duplicate, which sounds backwards until you realise the cylinder has worn to fit that one worn key.
Another common one is a lock that turns from inside but not outside on a double cylinder deadbolt, which usually means debris or a broken pin on the outside plug rather than a failed bolt.
Repair beats replacement more often than customers expect. New springs and pins, a clean out with a dry lubricant and a fresh key to code will restore most residential cylinders. Never use an oil based lubricant inside a cylinder. It collects dust and turns into a paste that guarantees a return visit.
Rekeying
Rekeying changes which key operates a lock without changing the lock. It is the single most requested residential service, it costs almost nothing in parts, and it is the correct answer to most of the reasons people call a locksmith about their house.
- Use a follower and keep constant contact so no top pin drops into the plug.
- Test four times. Cylinder alone, installed, door open, door closed.
- Keying several doors alike removes the reason people prop a door open.
- Tool rekeyable cylinders use sliding wafers, so conventional picking and pinning do not apply.
The procedure
Remove the cylinder, retain the plug with a follower so the top pins and springs stay in the housing, and slide the plug out. Tip the old bottom pins out, insert the new key, and fit the bottom pin that brings each stack level with the top of the plug. Slide the plug back in behind the follower, keeping constant contact so no top pin drops into the plug, and reassemble.
Test before the door closes. Turn the new key several times in both directions in the cylinder alone, then in the installed lock, then with the door open, then with the door closed. Four tests, in that order. Skipping the last one is how a customer ends up locked out by the lock you just serviced.
Count your pins in and out. A pin left on the bench is a pin that is not in the lock, and a five pin lock running on four is a lock that opens to far more keys than it should.
When rekeying is the right answer
New house, lost key, ended tenancy, ended relationship, contractor finished, cleaner left, key given out years ago and never returned. In all of these the hardware is fine. Only the key list is wrong. Rekeying solves it for the price of a few pins.
Rekeying also lets you key several locks alike. A house with a front deadbolt, a back door knob and a garage entry on three different keys is a house where somebody props a door. One key on all three is both more convenient and more secure, and it is a quick upsell that customers genuinely thank you for.
Replace rather than rekey when the hardware is failing, when the grade is inadequate for the door, or when the keyway is so common that key control is impossible. Say which of those applies and why. A customer who understands the reason accepts the higher price.
Smart keyed and tool rekeyable cylinders
Kwikset SmartKey and similar systems let a homeowner rekey without disassembly. Insert the working key, turn a quarter, insert a small tool, remove the old key, insert the new key, turn back. The cylinder learns the new key.
These use a sliding wafer mechanism rather than a conventional pin stack, so standard picking does not apply and standard rekeying does not either. They also have their own failure modes. Forcing the mechanism or losing the reset sequence can lock the cylinder into a state that requires replacement.
Know how to reset them, carry the tools, and be honest about their security. They resist picking well and they are convenient. Early versions had documented bypass weaknesses. For a customer whose main concern is a departed tenant they are entirely sensible. For a customer with a genuine targeted threat, sell a higher grade cylinder.
Deadbolts and Grading
Customers ask which lock is best. The honest answer is that the lock is rarely the weak point. The strike, the frame and the screws are. Grading gives you a defensible way to talk about quality, and understanding door reinforcement gives you something worth selling beyond the lock itself.
- Grade 1 survives ten hammer strikes and 250,000 cycles. Grade 3 survives two and 100,000.
- A deadbolt needs a one inch throw to reach meaningful material in the frame.
- Four three inch screws through the jamb into the stud beat a lock upgrade for stopping a kick.
- Double cylinder deadbolts trap people in fires. Check code and offer alternatives first.
ANSI and BHMA grades
The American National Standards Institute and the Builders Hardware Manufacturers Association publish a three tier grading standard. Grade 3 is basic residential. Grade 2 is heavy residential or light commercial. Grade 1 is commercial and the highest of the three.
The grades are earned on measured tests. A Grade 1 deadbolt survives ten strikes from a hammer test and two hundred and fifty thousand operating cycles. Grade 2 survives five strikes and one hundred and fifty thousand cycles. Grade 3 survives two strikes and one hundred thousand cycles. Those are the numbers behind the marketing.
Most homes ship with Grade 3. Moving a front door to Grade 1 is a real improvement and costs less than most customers assume. Say the numbers out loud. A customer choosing between a forty dollar lock and a ninety dollar lock deserves to know that one takes five times the beating.
Throw, strike and the three inch screw
A deadbolt should have a one inch throw. Anything less does not reach far enough into the frame to matter. Check it by extending the bolt and measuring from the face of the door.
The strike plate is where most residential doors fail. A standard strike is a thin plate held by two three quarter inch screws that bite only into the door jamb, which is a thin board nailed to the framing. A kick transfers straight through it and splits the jamb.
A reinforced strike box with four three inch screws driven through the jamb and into the structural stud behind it changes the failure mode entirely. The lock is no longer holding a decorative board. It is holding the house. This single upgrade costs a few dollars in parts and does more for a door than upgrading two grades of lock.
Single cylinder against double cylinder
A single cylinder deadbolt takes a key outside and a thumbturn inside. A double cylinder takes a key on both sides. Customers with glass beside the door often ask for a double cylinder so an intruder cannot reach through and turn the thumbturn.
The trade is escape. In a fire, in smoke, in the dark, a double cylinder deadbolt requires a key to get out. Many jurisdictions restrict them on residential exits for exactly that reason and building codes should always be checked before fitting one.
There are better answers to glass beside a door. Security film on the glazing, a laminated glass panel, or a thumbturn that can be locked with a key when the house is empty and left free when it is occupied. Offer those first, explain the escape risk plainly, and put the customer's decision in writing.
Doors and Frames
A locksmith who only looks at the lock will keep being called back. Most residential security failures happen at the door, the frame or the hinge, and being able to see them turns a fifteen minute rekey into a properly secured entrance.
- A gap wider than an eighth of an inch is enough for a pry bar or a shim.
- A hollow core door sounds like a drum and cannot hold a bolt whatever lock you fit.
- Outward opening doors need non removable hinge pins or a security stud hinge.
- Walk the whole property. The sliding door and the garage service door are entrances too.
Reading a door for weakness
Start at the gap. A gap wider than an eighth of an inch between door and frame gives room for a pry bar and often room for a shim past the latch. A sagging door that scrapes the sill has dropped on its hinges and its bolt is no longer aligned with the strike.
Check the door itself. A hollow core interior door fitted as an exterior door will fail around the lock no matter what lock you fit, because the bolt is anchored in a cardboard honeycomb. Tap it. A hollow core sounds like a drum.
Check the frame for splits, especially around the existing strike. Old damage that was painted over is a repair the customer does not know they need, and pointing it out before it fails is the difference between a technician and a salesperson.
Hinges and outward opening doors
On a door that opens outward the hinge pins are on the outside. Anyone with a screwdriver can drive the pins out and lift the door off from the hinge side, leaving the lock entirely irrelevant.
The fix is a set of security hinges with non removable pins, or a hinge with a fixed stud on one leaf that drops into a hole in the other so the door cannot be lifted even with the pins gone. Both are inexpensive and both take minutes.
On any door, replace at least two hinge screws per hinge with three inch screws into the stud. Hinges fail in a kick as often as strikes do, and nobody reinforces them because nobody looks at them.
Sliding doors and secondary entrances
Sliding patio doors are the entrance most often ignored and most often used. The factory latch is usually a light hook into a thin aluminium frame, and many can be lifted out of the track entirely.
Fit an anti lift device or adjust the rollers so the panel cannot be raised, then add a secondary lock. A pin lock through the frame at the top of the fixed panel is far stronger than a bar in the track, and it can be positioned to allow a ventilation gap.
Walk the whole property when you are already there. Garage service doors, basement hatches, gates and pet doors are all entrances. A pet door large enough for a person is a door, and quite a few of them are.
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