Guide
What Is Inside a uPVC Door Lock Mechanism, and What Goes Wrong
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uPVC Door Lock Parts, Named
Open a uPVC door and you are looking at three components, not one, and they are easy to mix up. Running the height of the door edge is the strip, with its locking points. Set into the middle of it sits the gearbox that drives them. Passing through both is the cylinder, which takes the key. Householders and quotes alike use the word "lock" for any of the three, and that is why prices for one door can differ so widely.
The Door Edge Strip and Its Locking Points
Trade suppliers call the strip the multipoint mechanism. Its visible face, the metal band you can run a finger along, is the faceplate. Several kinds of locking point can come out of it:
- Hooks, which swing out and draw the door into the frame
- Rollers, round adjustable studs that control how tightly the door closes
- Mushrooms, with a domed head shaped to resist being prised
- Pinbolts, short bolts that slide straight out
- Deadbolts, larger bolts that also slide rather than swing
On the frame side, each point meets a keep, the slotted plate that receives it.
Gearbox, Cylinder and Handle
The handle drives a square spindle. That spindle locates in the follower, a square socket inside the gearbox, and the gearbox moves every point together. The euro cylinder is a separate barrel that passes through the mechanism and works the gearbox through a cam on its back. To place this among other locks, see the full range of UK door lock types.
The Four Kinds of uPVC Mechanism
Every uPVC mechanism falls into one of four supplier categories, namely full lock, gearbox centre case on its own, overnight lock and auto slam shut. That grouping is the one UPVC Spares 4 Repairs uses for its mechanism types, and it is best read as the way parts are filed rather than as a standard. The categories exist because the thing you buy and the way the door behaves are linked, so naming one usually narrows the other.
Full Lock and Gearbox Centre Case
These two describe scope. A full lock replaces the complete strip and has to match the door on every measurement and every locking point. A gearbox centre case swap changes only the unit that failed, leaving the strip in place, and it is only possible on mechanisms designed with a removable case.
Auto Slam Shut and Overnight Lock
These two describe behaviour. With an overnight lock, closing the door leaves it on the latch until the handle is lifted and the key turned. With an auto slam shut, the points engage as the door closes. Converting between them is a mechanism change.
| How it behaves | Repair meaning | |
|---|---|---|
| Full lock | As the fitted strip dictates | Full replacement of the strip |
| Gearbox centre case | Behaviour unaltered | Centre unit changed alone |
| Overnight lock | Latch, then handle, then key | Same type needed |
| Auto slam shut | Self-locking on closing | Same type needed |
A Mechanism, Identified and Matched
A uPVC mechanism is identified, and a replacement matched, on four measurements together with the number and type of locking points, and an error in any one of those means the wrong part arrives. The single most useful marker is the faceplate stamp. Manufacturers usually press their name into the faceplate, and knowing it narrows the rest of the search; it is a way of recognising the part, not a recommendation.
The Measurements, All Four
The four specifications and their ranges come from UPVC Spares 4 Repairs; the Home Secure changeover guide supports the backset and PZ definitions.
| Measurement | Definition | Stocked range |
|---|---|---|
| Backset | From the front of the faceplate to the euro cylinder cut out centre | Several sizes stocked |
| PZ | Between the cylinder centre and the spindle centre | From 48mm to 117mm |
| Faceplate width | The faceplate measured across | From 16mm to 45mm |
| Screw centres | The gap between the fixing screws | 215mm and 240mm most often |
Matching also depends on overall length, taken along the full strip.
Counting Up the Locking Points
Count with the handle lifted and the door open. Listings from the same supplier show hooks from none to five, rollers none, two or four, mushrooms none or two, pinbolts none or two, and deadbolts none to three. No combination is better than another; it is simply what the door has.
Write it all down: stamp, measurements, length and points, plus one photo of the edge with everything out.
Gearbox Alone, or the Whole Strip?
It is the mechanism, and not whoever is quoting, that settles whether the gearbox can come out on its own. Three points, taken in order, explain how.
- On a mechanism with a removable centre case, the gearbox is changed and everything else, hooks and faceplate included, stays in the door.
- On a mechanism without one, a gearbox fault means the full strip, matched on every one of the four measurements and every locking point used in the UPVC Spares 4 Repairs filters.
- The Home Secure changeover guide states that full multipoint locks are too long to ship and have to be matched and bought in person, while gearboxes are posted.
So a gearbox can often be sourced quickly, and a full strip usually means somebody at the door comparing the old strip in their hand with what is available. If you are told your door needs the whole strip, those are the reasons that decide it, and they are worth asking about.
The Usual Order in Which Things Fail
When a uPVC mechanism stops working, the gearbox is usually the culprit; the other parts on this list mostly wear or drift out of line rather than break. The reason is simple. Each lift of the handle loads the gearbox, so it takes more punishment than anything else in the door edge. Failure is also rarely sudden: a door that has turned stiff and heavy over a few weeks is often giving notice of exactly this.
| Part | Symptom to look for |
|---|---|
| Gearbox | A handle that has lost its spring or jams before the end of its travel |
| Follower and spindle | Slack or wobble in the handle that comes before failure |
| Rollers and keeps | A shove needed to lock, or only part of the strip engaging, from misalignment |
| Euro cylinder | A key fault, with the handle behaving normally |
| Faceplate screws | Seldom the cause, though a loose faceplate worsens the feel of everything |
Each symptom has its own guide: a uPVC door that will not lock, the door that will not open at all, or a key that will not turn at all.
What Keeps a Mechanism Working
Most gearbox wear is avoidable in one way: lift the handle completely every single time you lock up. When the handle only goes half way, the gearbox takes the load yet never finishes its travel. Beyond that habit, keep to a short list.
Lubricate where the movement is.
That means the points along the door edge, using a product meant for locks, and not the keyhole.
Keep the frame side clean.
The keeps and the threshold need to stay clear so nothing obstructs the points.
Do not ignore stiffness.
A door that has become harder to lock is giving notice, and that is the best moment to deal with it.
Keep weight off the handle.
Hanging anything heavy there puts a constant strain on the spindle.
Temperature changes through the year can shift a door's alignment as well. Maintenance buys time rather than immunity, because a gearbox is a wearing part by design. The seasonal figures sit in the guide on a uPVC door that will not lock.
Your Mechanism, Identified
Identifying the mechanism is where every uPVC lock job begins, and it is the help we offer first. Phone Wilds Locksmiths Farnborough and describe what the faceplate stamp says and what you see along the edge with the handle lifted; usually that lets us name the mechanism and tell you whether the gearbox comes out on its own.
For a Quicker Call
- read the stamp on the faceplate aloud
- give any of the four measurements you have
- explain what the handle does when you lift it
You will not pay a call-out charge at any hour. The phones are answered every day of the year, nights included. Pricing happens on the call from your description, and if the work changes once someone is at the door, you are told before it starts.
Describe It and Get It Identified: 01252 493213
Or call or message Wilds Locksmiths Farnborough to ask about 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?
Yes, very often the gearbox is the part to suspect. The important distinction is between a likely explanation and an established fault. Get a qualified locksmith to check the door and mechanism before selecting parts; the symptoms provide a starting point for that assessment.
Can I just replace the gearbox?
The centre case must be removable for the gearbox to be replaced on its own. Do not assume that a suspected gearbox fault means a separate replacement is possible. First establish the mechanism's construction, with a qualified locksmith's help if needed, then choose the appropriate next step.
Is the cylinder part of the mechanism?
No, the mechanism does not include the cylinder. Refer to each part by its own name when explaining the problem. If a replacement has been suggested, clarify which of the two is involved before selecting anything. A qualified locksmith can help make that description clear.
How do I find out which mechanism I have?
Lift the handle and count the locking points, take the four measurements, and check the faceplate for a stamped manufacturer name. Note the results together so the mechanism can be identified from the full set of details. Include the name as it appears on the faceplate.
Why can my door not be left on the latch?
It is likely to be an automatic slam-shut system. With that mechanism, the door's closing action throws the locking points, accounting for why the door is not remaining on its latch as expected. Confirm the type fitted before deciding whether the behaviour indicates a problem.
Why does a full strip take longer to source than a gearbox?
Matching and buying the full strip in person can make sourcing take longer. Home Secure puts this down to the strip being too long to ship. The delay therefore reflects the process of obtaining a matching complete strip, rather than simply choosing the gearbox as a separate part.
How long should a uPVC mechanism last?
It cannot be assigned a set service period. With no fixed lifespan for the mechanism, a guessed number of years is not a sound replacement rule. Have a qualified locksmith examine the fitted unit if a decision is needed about whether it should be changed.