Guide
Urgent Fault or Planned Fix? What Is Inside a uPVC Door Lock Mechanism, and What Goes Wrong
If your uPVC door has started to stiffen, slip or refuse to lock, it helps to know that the lock on it is not one part. It is a metal strip running the height of the door edge, with a gearbox in the middle of it and a cylinder passing through it, and each of those can fail on its own.
This guide from Runcorn Locksmith names every part you can see or feel, sorts mechanisms into the four types the trade uses, sets out the measurements a replacement is matched on, and explains which part usually goes first. If the door will not secure tonight, emergency calls are taken 24 hours a day, every day of the year, on 01928 611001. If it can wait, read on, because knowing the names saves time and money on the call.
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The Parts of a uPVC Door Lock, Named
There are three things stacked into a uPVC door, and people mix them up constantly. The first is the strip in the door edge with its locking points. The second is the gearbox that drives them. The third is the cylinder that takes your key. All three get called "the lock", which is one reason quotes for the same door can vary so widely: one person means a cylinder, another means the whole strip. The full range of other lock types is covered in the full range of UK door lock types; this page sticks to the uPVC multipoint system.
| Part | Where it is | What it does |
|---|---|---|
| Multipoint mechanism | Inside the door edge, full height | Carries every locking point |
| Faceplate | The flat metal band visible on the edge | Covers and fixes the mechanism |
| Hook | Along the faceplate | Swings out and pulls the door into the frame |
| Roller | Along the faceplate | Round and adjustable; sets how tightly the door compresses |
| Mushroom | Along the faceplate | Shaped to resist levering |
| Pinbolt | Along the faceplate | Slides straight out into the frame |
| Deadbolt | Along the faceplate | Slides straight out, rather than swinging |
| Keep | On the frame | The slot each locking point engages with |
| Gearbox (centre case) | Middle of the strip | Converts handle movement into locking-point movement |
| Spindle | Through the door, between handles | Carries the handle's turn into the gearbox |
| Follower | Inside the gearbox | The square hole the spindle sits in |
| Euro cylinder | Through the gearbox | Takes the key and turns a cam |
The Strip in the Door Edge and Its Locking Points
The strip is the multipoint mechanism itself, and the flat metal band you can see along the edge of an open door is its faceplate. Open the door, lift the handle, and every locking point shows itself. Each type has a different job:
- Hooks swing out and draw the door tight into the frame.
- Rollers are round and adjustable, and they decide how hard the door presses against its seals.
- Mushrooms have a wide head designed to resist the door being levered away from the frame.
- Pinbolts slide straight out rather than swinging.
- Deadbolts also slide straight out instead of swinging.
Doors carry different combinations of these, and that combination is part of how a replacement is specified, which the identification section below explains. The locking point categories and their count ranges used here come from the mechanism filters published by UPVC Spares 4 Repairs. The last thing to name is the keeps: the matching slots on the frame. Half the faults described further down come down to a locking point missing its keep rather than anything breaking.
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 once.
| Part | What moves it | What it moves |
|---|---|---|
| Handle | Your hand | The spindle |
| Spindle | The handle, via the follower | The gearbox |
| Gearbox | The spindle, or the cylinder's cam | Every locking point |
The cylinder sits apart from all of that, and this is the confusion that costs people money. It is the barrel the key goes into. It passes through the mechanism rather than being part of it, and it works the gearbox by turning a small cam. So a key fault and a handle fault point to different parts, and a cylinder can be changed without disturbing the mechanism at all, as covered in changing the cylinder without touching the mechanism. The follower, the square hole in the gearbox that the spindle sits in, is a common wear point. Put simply, three different parts can each leave you with a door that will not lock, and they are three very different jobs.
The Four Kinds of uPVC Mechanism
Parts suppliers sort mechanisms into four groups: a full lock, a gearbox centre case on its own, an overnight lock and an auto slam shut. UPVC Spares 4 Repairs lists its mechanisms under exactly these four headings. They are how the trade sorts stock rather than an official standard, and they describe two linked things at once: what you can buy, and how your door behaves when it closes.
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 is the difference between two very different jobs.
| Full lock | Gearbox centre case |
|---|---|
| Everything in the door edge comes out | The strip and all its locking points stay in the door |
| The new strip must match every measurement and locking point | Only the failed driving unit is changed |
| Needed when the mechanism is not designed to separate | Possible only where the centre case is removable |
A full replacement takes out everything in the edge and fits a matched strip in its place. A centre case swap leaves the hooks, rollers and faceplate where they are and changes only the unit in the middle, which is usually the part that failed. The catch is that not every mechanism is built so the centre case comes out on its own, and on those that are not, a gearbox fault means a whole strip. Which one you have is a property of the mechanism, not a choice anybody makes for you, and the identification section 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 moment it shuts, you have the other.
- Overnight lock. The usual pattern on UK front doors. Pull it closed and it is only on the latch. Lift the handle to throw the hooks, then turn the key to lock them.
- Auto slam shut. The locking points are thrown as the door closes, with no handle needed, which is why some doors cannot be left on the latch.
The day-to-day consequence is worth knowing: on an auto slam shut door it is much easier to end up outside without your keys, because the door secures itself behind you. If that has just happened, see getting back in after a door has locked itself. The two types are not interchangeable, and turning one into the other is a mechanism change rather than an adjustment.
| Type | How the door behaves | What it means for a repair |
|---|---|---|
| Full lock | Depends on the design | The entire strip is replaced as a matched unit |
| Gearbox centre case | Depends on the design | Only the central driving unit is swapped, if the mechanism allows it |
| Overnight lock | Latches when shut; lift the handle, then turn the key | The familiar pattern; parts must match this behaviour |
| Auto slam shut | Locks itself as it closes, without the handle | Easy to get locked out; cannot be converted without a new mechanism |
None of these is better than another in general. Your door has the type it has, and changing type means changing the mechanism.
How a Mechanism Is Identified and Matched
A multipoint mechanism is matched on four measurements plus the number and type of its locking points, and getting any one of them wrong means the wrong part. The shortcut that narrows everything is the manufacturer name, which is usually stamped on the faceplate. Look along the edge with the door open; the stamp is an identification marker, not a sign of quality, and no one maker is being recommended here. The specifications below are the filter definitions used by UPVC Spares 4 Repairs, and the backset and PZ definitions are corroborated 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 centre of the spindle | 48mm up to 117mm |
| Faceplate width | Width of the visible metal band | 16mm to 45mm |
| Screw centres | Distance between fixing screws | Commonly 215mm and 240mm |
The ranges are the point. A reader who sees that PZ alone runs from 48mm to 117mm understands at once why guessing does not work. These are what suppliers stock, not a standard. One thing in the sources is worth flagging: one supplier measures backset from the front of the faceplate and another from the edge of the gearbox case. On most mechanisms those are the same plane, but use the faceplate front, because that is what stock is filtered on. Overall length has to match as well, measured end to end along the strip.
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 people.
| Locking point | Counts suppliers stock |
|---|---|
| Hooks | None up to five |
| Rollers | None, two or four |
| Mushrooms | None or two |
| Pinbolts | None or two |
| Deadbolts | None up to three |
These are UPVC Spares 4 Repairs' own filter values. Two doors that look the same from the street can carry completely different strips. No combination is better or more secure in itself; it is simply what the door was built with. Write the counts down, or photograph the open edge with the handle lifted. That photograph plus the faceplate stamp is most of what anyone needs to identify the part, which is why a locksmith asks for a picture rather than the brand of the door. If you are also thinking about security, see what a rated cylinder adds to any of these.
Checklist to photograph or write 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
- Overnight lock or auto slam shut
What Fails, and Usually in What Order
The gearbox is the part that genuinely fails; almost everything else is wear or alignment rather than breakage.
| Part | Typical symptom |
|---|---|
| Gearbox | Handle goes slack or jams partway; the most common mechanism fault, because it takes the load of every lift |
| Follower and spindle | Handle feels vague or has play in it before it fails |
| Rollers and keeps | Door needs a shove, or only some points engage; alignment rather than breakage |
| Euro cylinder | The key is the problem, not the handle |
| Faceplate screws | Rare; a loose faceplate makes everything feel worse than it is |
Most mechanism failures give notice. Weeks of stiffness usually come first, so a door that has become hard work is telling you something. This page catalogues the faults; three sibling guides diagnose them properly: the door that will not open at all, a key that will not turn at all, and the door that will not lock.
Gearbox Alone, or the Whole Strip?
Whether the gearbox can be changed on its own is decided by the mechanism, not by whoever is quoting. Three things settle it:
A removable centre case.
Where the mechanism is built with one, a gearbox swap leaves every hook and the faceplate in place.
No removable centre case.
Then a gearbox fault means a full strip, and that strip has to match on all four measurements and every locking point.
Shipping length.
Home Secure states that full multipoint locks are too long to ship, so they have to be matched and bought in person, while gearboxes can be sent by post.
That last point explains a lot. A gearbox can often be sourced quickly. A full strip usually means someone standing at the door with the old one in hand, matching it. Nobody is choosing the larger job for you; the mechanism decides it. Whether parts are carried or sourced on the day: [CLIENT TO CONFIRM: van stock and whether mechanisms are sourced same day].
What Keeps a Mechanism Working
The single most useful habit: lift the handle fully every time you lock. A gearbox driven halfway takes the strain without finishing its travel.
Lubricate the moving parts in the door edge
not the keyhole, using a product made for locks.
Keep the keeps and threshold clear
so locking points meet their slots cleanly.
Deal with stiffness when it starts.
Stiffness is the notice period, and seasonal swelling is explained in our door will not lock guide.
Do not hang heavy things on the handle
which loads the spindle and follower.
Maintenance delays a gearbox failure; it does not prevent one, because the part wears by design.
Get Your Mechanism Identified
Tell us what the faceplate stamp says, what the locking points are and what the handle is doing, and we can usually name the mechanism and say whether the gearbox comes out on its own. Have ready the four measurements if you have taken them, the locking point count and a description of the fault [CLIENT TO CONFIRM: whether photographs are accepted by WhatsApp or email].
There is no call-out charge at any hour, calls are taken 24 hours a day, every day of the year, and the price is given on the call from what you describe, with any change explained before work begins. Technicians: [PROVIDER PACK: DBS and insurance evidence].
- Describe It and Get It Identified: 01928 611001
- Call or message Runcorn Locksmith
- Often it is a lock repair rather than a full replacement.
uPVC Mechanism Questions
My handle has gone floppy and the door will not lock. Is that the gearbox?
Very often, yes. A handle that goes slack or stops partway is the classic gearbox symptom. Worn followers and spindles can feel similar, so the fault is confirmed by inspection, but the gearbox is the most likely place to start.
Can I just replace the gearbox?
Only if your mechanism has a removable centre case. Where it does, the strip and locking points stay in the door. Where it does not, a gearbox fault means a full matched strip.
Is the cylinder part of the mechanism?
No. The euro cylinder passes through the gearbox and drives it with a cam, but it is a separate part. It can be changed without touching the mechanism, which is why a key problem and a handle problem lead to different repairs.
How do I find out which mechanism I have?
Look for the manufacturer name stamped on the faceplate, take the four measurements, and count the locking points with the handle lifted. Note whether your door locks itself as it shuts. Together, those identify most mechanisms.
Why can my door not be left on the latch?
It probably has an auto slam shut mechanism, which throws its locking points as the door closes. Converting it to an overnight lock is a mechanism change, not an adjustment.
Why does a full strip take longer to source than a gearbox?
Full strips are long, and Home Secure notes they are too long to ship, so they are matched and bought in person. Gearboxes can be posted, which usually makes them quicker to get hold of.
How long should a uPVC mechanism last?
There is no fixed lifespan. It depends on how often the door is used, whether the handle is lifted fully each time, and how well the door is aligned. Good habits extend the life of the gearbox, but it is a wearing part and will eventually need replacing.