How a Lock Actually Works

The Mechanism, in Plain Terms

A cylinder has two parts: an outer housing and an inner plug that rotates inside it. The plug is what turns to move the bolt, and it cannot rotate unless something releases it.

That something is a row of small pins. Each pin is in two pieces — a lower one that touches the key and an upper one pushed down by a spring. With no key in, the upper pins straddle the join between the plug and the housing, which locks the two together and prevents any rotation.

The correct key lifts every pin to exactly the height where the join between its two pieces lines up with the join between plug and housing. At that instant nothing is straddling the boundary and the plug is free to rotate. That alignment is the entire secret of the lock.

That is why a key must be precisely shaped rather than approximately shaped, and why a worn key eventually stops working: it is no longer lifting the pins to quite the right heights, which is what our locksmith for lock change page means when it says a replacement has to match the door rather than the previous product.

Four Ways That Mechanism Fails

Every common lock fault is one of these, and telling them apart is the whole trade.

Wear. Pins, springs and the plug wear microscopically with every use, and after years the tolerances that made the alignment precise become sloppy. The key needs a wiggle, then a particular angle, then two attempts on a cold morning.

Contamination. Dust and grit accumulate around the pins and in the springs. Add oil — which people spray in as a remedy — and the mixture becomes grinding paste that accelerates the wear it was meant to relieve. This is the single most common self-inflicted lock injury, and our locksmith for lock repair in Sandton technicians ask about it before touching anything.

Alignment. Nothing wrong with the cylinder at all: the door has moved and the bolt no longer meets the keep squarely, so the mechanism is being asked to work under load. Replacing the cylinder changes nothing, which our residential locksmith visits demonstrate by locking the door while it stands open.

Damage. Something has been forced. Pins deform, the plug is scored, springs are crushed. This is the one case where a repair genuinely will not hold.

Six Symptoms, Mapped to the Mechanism

What is physically happening inside, for each of the common complaints.

What a Repair Physically Involves

Knowing the mechanism makes it obvious why a repair frequently works, and why it sometimes cannot.

The lock comes off and is opened. The plug is separated from the housing, the pins and springs come out, and everything is cleaned properly — which for a contaminated lock is most of the fix on its own. Worn pins and tired springs are replaced. The correct lubricant, which does not hold dust, is applied sparingly.

Reassembled, a mechanism that was two months from seizing is frequently back to working properly and will run for years. That is a service rather than a rebuild, and it is why so many quoted replacements are unnecessary.

Where it will not hold is when the housing is cracked, the plug is scored from forcing, or the pattern is genuinely obsolete and parts do not exist. In those cases a replacement is right, and our lock replacement page sets out the three situations where that is genuinely the case.

Why Some Locks Cost More Than Others

Once the mechanism is clear, the price differences between cylinders stop looking arbitrary.

A basic cylinder has a single row of pins and reasonable tolerances. A higher-grade one adds things that make the alignment harder to achieve by any means other than the correct key: more pins, pins of awkward shapes, a second row acting on a different plane, and hardened inserts that resist drilling.

There is also anti-snap design, which addresses a completely different attack — not defeating the pins at all, but breaking the cylinder in two and manipulating what is left. A cylinder built with a sacrificial break line fails in a way that leaves the mechanism intact, which is what our locksmith for lock installation assessments consider on exposed doors.

None of that helps if the opening was never the weak point, which is the argument our lock replacement page makes for spending on the weakest door rather than the strongest lock.

How Rekeying Works, Given the Mechanism

This is the part that surprises people most, and it follows directly from how the pins work.

1. Step 1

   The cylinder stays in the door

   Nothing about the housing, the bolt or the fixings changes. Only what is inside the plug does.

2. Step 2

   The plug is removed and the pins come out

   The existing pin heights are what make the current key work, and they are the only thing being changed.

3. Step 3

   Different pin heights are fitted

   A new combination of lower pins, matched to a different key. The alignment now happens for a different shape.

4. Step 4

   Every previous key stops working

   Immediately and permanently, because those keys no longer lift the pins to the new heights.

5. Step 5

   You get a counted new set

   Which is the entire point: the number of working keys becomes known again without replacing any hardware.

What This Lets You Judge for Yourself

Four claims you can now evaluate without knowing anything else about locks.

“It needs replacing because it is old”

Age is not a fault. Wear and contamination are, and both are frequently repairable. Ask which one it is.

“The cylinder has failed” on a door that binds

If it locks fine while standing open, the cylinder has not failed. The door has moved and new hardware will not help.

“You need new locks after losing keys”

You need rekeying. The hardware is fine; only the pin heights need changing, and that keeps the lock in the door.

“It still works so it is fine” after forcing

Deformed pins and a scored plug do not announce themselves. Forced hardware is the one case for replacement.

Why Oil Makes a Stiff Lock Worse

The universal response to a stiff lock is to spray something into it, and it is the single most damaging thing routinely done to domestic hardware.

General-purpose penetrating oil does free a mechanism, and for a week or two it works convincingly. What follows is the problem. Oil in a keyway is a surface that collects everything that subsequently enters it — dust, grit, the fine debris that gets carried in on a key — and it holds that material in suspension against the moving parts. Within a month or two the mechanism is binding considerably harder than it was before anybody intervened.

Precision and high-security cylinders are markedly less tolerant of this than cheap ones, because their internal clearances are tighter and there is less room for anything that should not be in there. A cylinder designed to run dry, or on a specific dry lubricant, is not improved by a general-purpose oil.

The other reason it matters is diagnostic. A lock that has been oiled presents differently from one that has not, and it can mask the symptom that would otherwise have identified the actual fault.

So a stiff lock is a reason to have it opened up and cleaned properly rather than a reason to reach for a can. And if one has already been oiled — which is extremely common and nothing to be embarrassed about — saying so on the call changes what needs doing.

A Door Is an Assembly, Not a Lock

Almost every misdiagnosis in this trade comes from treating the lock as the thing and the door as the setting. It is the other way round.

A door is an assembly: the leaf, the frame, the hinges, the strike plate and the fixings holding all of it together. Every one of those carries load, and the assembly performs at the level of its weakest component rather than its best one. That single idea explains more failures than any amount of detail about mechanisms.

It explains why a better lock frequently changes nothing. A substantial mechanism in a light door concentrates force into material never intended to take it, so the lock becomes excellent and the door around it becomes the failure point.

It explains the thirty-second test. If a key turns freely with the door open and fights with it closed, the mechanism is sound and the bolt is no longer meeting its keep — that is a hinge that has dropped, timber that has moved with the season, or a frame that has settled, and no replacement lock corrects any of them.

And it explains why upgrading a cylinder while leaving the original strike plate and its short screws in place moves the weak point rather than removing it: the bolt is now stronger than the thing it engages, and force simply finds the fixings instead.

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