Guide
Woking uPVC Door Lock Mechanisms: What Is Inside, and What Goes Wrong
Call 01483 377197 if your door will not open or will not lock right now; if it is simply getting stiff, this guide will help you understand it before you book anything.
The lock on a uPVC door is not a single part. It is a metal strip that runs the full height of the door edge, with a gearbox set into the middle of it and a cylinder passing straight through. Most of the confusion around uPVC lock faults, and most of the variation in quotes, comes from treating those pieces as one thing.
This page names every part, sorts mechanisms into the four kinds the parts trade uses, lists the measurements a replacement has to match, and explains what tends to fail first. Manufacturers come up only as names stamped on the metal, to help with identification. If you would rather someone looked at your own door, Woking Locksmith can talk it through on the phone.
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The Parts of a uPVC Door Lock, Named
Three separate things are built into a uPVC door, and they are routinely mixed up. The first is the strip with its locking points. The second is the gearbox that drives those points. The third is the cylinder, the barrel your key goes into. People call all three "the lock", which is exactly why two quotes for the same door can look nothing alike. For the wider family of locks found on UK doors, see the full range of UK door lock types.
The list below gives each component its trade name in a single line, so you can describe your own door accurately to anyone.
| Part | What it is |
|---|---|
| Multipoint mechanism | The full-height strip in the door edge that carries every locking point |
| Faceplate | The flat metal band you can see along the edge of the door |
| Gearbox (centre case) | The driving unit in the middle of the strip, behind the handle |
| Euro cylinder | The key barrel, which passes through the mechanism |
| Cam | The small lever on the cylinder that turns the gearbox |
| Spindle | The square bar that links the handle to the gearbox |
| Follower | The square hole in the gearbox that the spindle sits in |
| Hook | A locking point that swings out and draws the door into the frame |
| Roller | A round, adjustable locking point that sets how tightly the door seals |
| Mushroom | A locking point shaped to resist levering |
| Pinbolt / deadbolt | Locking points that slide straight out rather than swinging |
| Keep | The matching slot on the frame that each locking point engages |
The Strip in the Door Edge and Its Locking Points
The strip itself is the multipoint mechanism, and the flat band covering it along the door edge is the faceplate. Open the door and lift the handle, and you can see every locking point it carries. The parts suppliers UPVC Spares 4 Repairs sort these into five kinds:
- Hooks swing outwards and pull the door tight into the frame.
- Rollers are round and adjustable, and they decide how hard the door compresses its seals.
- Mushrooms are shaped so the door resists being prised.
- Pinbolts slide straight out of the edge.
- Deadbolts also slide straight out, without swinging.
Doors carry different mixtures of these, and the mixture is part of how a replacement is specified, which the identification section explains. Each point closes into a keep, the matching slot on the frame, and a good share of the faults further down this page come from a point that has stopped meeting its keep cleanly.
The Gearbox, the Cylinder and the Handle
Follow the force. The handle turns a spindle. The spindle turns the gearbox. The gearbox moves every locking point at the same moment.
The cylinder sits outside that chain, and this is the mix-up that costs people money. It is the barrel the key goes into; it passes through the mechanism rather than forming part of it, and it works the gearbox by turning a small cam. So a fault with the key and a fault with the handle point to different parts, and a cylinder can be swapped without touching the mechanism at all, as the guide on changing the cylinder without touching the mechanism explains. The follower, the square socket the spindle sits in, is a common place for wear to show.
| Part | Moved by | Moves |
|---|---|---|
| Handle and spindle | Your hand | The gearbox, via the follower |
| Gearbox | Spindle or cylinder cam | Every locking point on the strip |
| Euro cylinder | The key | The gearbox, via its cam |
The consequence is worth spelling out: three different parts can each give you a door that will not lock, and they are three very different jobs.
The Four Kinds of uPVC Mechanism
The trade groups uPVC mechanisms into four kinds: a full lock, a gearbox centre case supplied on its own, an overnight lock and an auto slam shut. These are the categories UPVC Spares 4 Repairs lists its stock under, so treat them as the way the parts trade sorts mechanisms rather than as a formal standard. They describe both what you can buy and how the door behaves as it closes, and the two are connected.
Full Lock and Gearbox Centre Case
A full lock is the entire strip; a gearbox centre case is only the driving unit from the middle of it. That makes them two very different jobs.
| Full lock | Gearbox centre case |
|---|---|
| Everything in the door edge comes out | The strip, hooks and faceplate stay in the door |
| The new strip must match every measurement and locking point | Only the failed driving unit is changed |
| Needed when the centre case cannot come out alone | Possible only on mechanisms built for it |
A centre case swap leaves everything else in place and replaces just the part that has failed, and that is usually the part that does fail. The catch is that not every mechanism is built so its centre case can be removed separately. On those that are not, a gearbox fault means a full strip. Which arrangement you have is a property of the mechanism, not a choice anyone makes for you, and the identification section below is how you find out.
Overnight Lock and Auto Slam Shut
If your door needs the handle lifted before the key will turn, you have one kind. If it latches and locks itself the instant it closes, you have the other.
- Overnight lock. The pattern on a great many UK front doors. Pull it shut and it is only on the latch; lift the handle to throw the hooks, then turn the key to lock.
- Auto slam shut. The locking points throw as the door closes, with no lift of the handle, which is why some doors simply cannot be left on the latch.
That second behaviour has a practical consequence: with an auto slam shut, it is far easier to lock yourself out, because the door secures itself behind you. If that has just happened, see getting back in after a door has locked itself. The two kinds are not interchangeable, and converting a door from one to the other means changing the mechanism.
| Type | How the door behaves | What it means for a repair |
|---|---|---|
| Full lock | Depends on the mechanism | The whole strip is replaced and must match on every measurement |
| Gearbox centre case | Depends on the mechanism | Only the driving unit is changed; the strip stays |
| Overnight lock | Latches on closing; lift the handle, then turn the key | The familiar pattern; parts must match that behaviour |
| Auto slam shut | Locks itself as it closes | Cannot be left on the latch; not interchangeable with overnight |
Which kind your door has is fixed by its mechanism, so there is no sense in which one is the right choice.
How a Mechanism Is Identified and Matched
A multipoint mechanism is matched on four measurements together with the number and type of its locking points, and getting any one of them wrong means the wrong part. One clue shortens the whole job: the manufacturer's name is usually stamped on the faceplate, and once you have it, everything else narrows quickly. The definitions here follow the filters UPVC Spares 4 Repairs uses for its stock, with the backset and PZ wording also supported by Home Secure's changeover guide.
The Four Measurements
The four are backset, PZ, faceplate width and screw centres.
| Measurement | What it measures | What suppliers stock |
|---|---|---|
| Backset | Front of the faceplate to the centre of the euro cylinder cut-out | Varies by mechanism |
| PZ (spindle to key) | Centre of the cylinder to the spindle | 48mm up to 117mm |
| Faceplate width | Width of the band along the door edge | 16mm to 45mm |
| Screw centres | Spacing between fixing points | Commonly 215mm and 240mm |
Those ranges show why guessing does not work: PZ alone spans 48mm to 117mm. They describe what suppliers carry, not a standard every door follows. One wrinkle in the sources is worth knowing. One supplier measures backset from the front of the faceplate and another from the edge of the gearbox case. On most mechanisms those sit in the same plane, but the faceplate front is the one to use, because that is what stock is filtered on. The overall length of the strip has to match as well, measured end to end.
Counting the Locking Points
Open the door, lift the handle, and count what comes out of the edge and what shape each piece is. That count is part of the specification, and the spread surprises most people.
| Locking point | Counts suppliers list |
|---|---|
| Hooks | None up to five |
| Rollers | None, two or four |
| Mushrooms | None or two |
| Pinbolts | None or two |
| Deadbolts | None up to three |
Those are UPVC Spares 4 Repairs' own filter values. They mean two doors that look identical from the street can carry completely different strips. The practical step is to photograph the open edge with the handle lifted. That photo, together with the stamp on the faceplate, is most of what anyone needs to identify the part, which is why a locksmith will ask for a picture rather than the make of door. No combination is better than another; it is simply what your door has. For what a rated cylinder adds whichever strip you have, see what a rated cylinder adds to any of these.
Specification checklist (photograph or note these down) - Manufacturer name stamped on the faceplate - Backset - PZ (spindle to key) - Faceplate width - Screw centres - Overall length of the strip - Number and type of locking points - Whether the door is overnight or auto slam shut
What Fails, and Usually in What Order
The gearbox is the part that genuinely fails; nearly everything else on the list is wear or alignment rather than breakage.
| Part | What you notice |
|---|---|
| Gearbox | The handle goes slack or jams part way; the most common mechanism fault, because it takes the load of every lift |
| Follower and spindle | Play in the handle, or a vague feel before it gives out |
| Rollers and keeps | The door needs a shove, or only some points engage: alignment, not breakage |
| Euro cylinder | The key is the trouble rather than the handle |
| Faceplate screws | Uncommon, but a loose faceplate makes everything feel worse than it is |
The pattern to remember is that most mechanism failures are preceded by weeks of stiffness. A door that has become hard work is giving you notice. This section only catalogues the parts; three other guides diagnose properly. For the door that will not lock, see why your uPVC door will not lock. For the door that will not open at all, and for a key that will not turn at all, follow those links.
Gearbox Alone, or the Whole Strip?
Whether the gearbox can be changed on its own is settled by the mechanism, not by whoever is quoting. Three things decide it.
A removable centre case.
Where the mechanism is built for it, a gearbox swap leaves every hook and the faceplate where they are.
No removable centre case.
Here a gearbox fault means a full strip, and that strip must match on all four measurements and every locking point.
Length.
Home Secure's changeover guide notes that full multipoint locks are too long to post and have to be matched and bought in person, while gearboxes can be sent by post.
That last point changes the picture. A gearbox can often be sourced fairly readily, whereas a full strip usually means someone standing at the door holding the old one so it can be matched. [CLIENT TO CONFIRM: whether mechanisms and gearboxes are carried or sourced, and how] None of this is about steering anyone towards the bigger job; it is simply what the hardware allows.
What Keeps a Mechanism Working
The single most useful habit is to lift the handle all the way every time you lock the door. A gearbox driven only part of the way takes the strain without finishing its travel.
Lubricate the moving parts in the door edge
not the keyhole, with a product made for locks.
Keep the keeps and the threshold clear
of grit and debris, so the points meet their slots cleanly.
Deal with stiffness as soon as it starts
because stiffness is the notice period.
Do not hang anything heavy on the handle
which loads the spindle and gearbox constantly.
Seasonal swelling also plays a part, and the will-not-lock guide covers it. Be realistic, though: looking after a mechanism postpones a gearbox failure, it does not prevent one, because the part wears by design.
Get Your Mechanism Identified
Describe the door edge with the handle lifted, read us the name on the faceplate, and we can usually tell you which mechanism it is and whether its gearbox comes out on its own. [CLIENT TO CONFIRM: whether photos can be received by WhatsApp or email; until confirmed, describe it on the call] Have ready what the handle is doing, the name on the faceplate, and the four measurements if you have taken them.
There is no call-out charge, whatever the hour, and calls are taken 24 hours a day, every day of the year, including nights, weekends and bank holidays. You get a price on the call from what you tell us, and if anything at the door changes it, you hear that before work starts. [PROVIDER PACK: DBS check evidence for technicians] [PROVIDER PACK: public liability insurance details]
Get It Identified: call 01483 377197
If it turns out to be a repair rather than a replacement, see lock repair rather than a full replacement, or uPVC door lock repairs for the wider service.
uPVC Mechanism Questions
My handle has gone floppy and the door will not lock. Is that the gearbox?
Very often. A handle that goes slack or jams part way through its lift is the classic gearbox symptom. A worn follower or spindle can feel similar at first, so it is worth having the door edge looked at before ordering anything.
Can I just replace the gearbox?
Only if your mechanism is built with a removable centre case. Where it is, the gearbox can be swapped and the strip stays in the door. Where it is not, a failed gearbox means a full strip that matches every measurement.
How do I find out which mechanism I have?
Look for a manufacturer's name stamped on the faceplate, then note the four measurements and count the locking points with the handle lifted. A photo of the open edge plus the stamp is usually enough to identify it.
Is my door an overnight lock or an auto slam shut?
If you lift the handle before the key will turn, it is an overnight lock. If the door locks itself the moment it closes, without the handle being lifted, it is an auto slam shut.
Why can a cylinder be changed without touching the mechanism?
Because the cylinder passes through the mechanism rather than being part of it. It operates the gearbox with a small cam, and it can be taken out and replaced on its own.
How long does a uPVC mechanism last?
There is no reliable single figure. Life depends on how often the door is used, whether it has stayed in alignment and whether the handle is lifted fully each time. Weeks of increasing stiffness are the usual warning before the gearbox gives out.
My door needs a shove before it will lock. Is the mechanism broken?
Probably not yet. A door that needs pushing usually has rollers or keeps out of line, which is alignment rather than breakage. Left alone, though, the extra strain on every lift wears the gearbox faster.