Guide
What Is Inside a uPVC Door Lock Mechanism, and What Goes Wrong
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Naming the Parts of a uPVC Door Lock
The first thing to know about a uPVC door lock is that it is three parts in one place, and they are regularly mistaken for each other. The locking points are carried by a strip in the door edge. That strip is driven by a gearbox. The key goes into a cylinder, which passes through the gearbox area. All three get called the lock, and that loose word is why quotes can vary so much for the same door in Bournemouth or anywhere else.
Locking Points and the Strip in the Door Edge
The multipoint mechanism is the strip itself, and its faceplate is the flat metal strip-cover you see with the door open. The locking points differ in how they move. A hook pivots out like a claw and drags the door towards the frame. A roller turns on its stud and can be adjusted to change how firmly the door closes. A mushroom has a widened head whose shape resists the door being forced apart. A pinbolt and a deadbolt travel straight out, with the deadbolt as the heavier relative. Fixed in the frame, a keep receives each point.
The Cylinder, the Gearbox and the Handle
The handle is attached to a spindle, the square shaft through the door. The follower is the gearbox's square socket for that shaft. Inside the gearbox, the turn becomes a push along the strip. The euro cylinder is the barrel for the key, passing through the mechanism as a separate part, and its cam is what turns the gearbox when you lock with the key.
The Four Kinds of Mechanism in uPVC
How does the trade classify a uPVC lock? Into four groups: full lock, gearbox centre case on its own, overnight lock and auto slam shut. Those are the mechanism types listed by UPVC Spares 4 Repairs, and they reflect how a supplier arranges its range, not a formal rule. Most homeowners have never heard two of the names, yet one of them almost certainly describes their own front door. Knowing which one you have tells you both what can be ordered and how your door locks.
Gearbox Centre Case and Full Lock
A full lock is the strip in its entirety. A centre case is the gearbox unit taken out of the middle, sold alone. Where a mechanism has no removable centre case, a gearbox fault means a whole strip.
Overnight Lock and Auto Slam Shut
Shut an overnight lock and it is merely latched; lifting the handle throws the points and the key locks them. An auto slam shut throws its points as it closes, so it cannot be left on the latch.
| Type | Behaviour | Repair implication |
|---|---|---|
| Full lock | Depends on the strip's design | The strip is replaced as one piece, matched to the door |
| Gearbox centre case | The door behaves exactly as it did | Driving unit only, where the mechanism permits |
| Overnight lock | On the latch until locked by handle and key | Replaced with another overnight lock |
| Auto slam shut | Locks itself on closing | Replaced with another auto slam shut |
None of the four is the right or wrong answer; each is simply what is in the door.
A Mechanism, Identified and Matched
Here is the rule that governs every replacement: a uPVC multipoint mechanism is matched on four measurements and on how many locking points it has and what kind they are, and one mistake means ordering the wrong part. Start with the faceplate, because the maker's name is usually stamped there. That one detail cuts the field down before a tape measure comes out, and it is used here purely to identify the part.
The Measurements, All Four
The four specifications below are the filter definitions of UPVC Spares 4 Repairs. The Home Secure changeover guide defines backset and PZ in the same terms.
| Specification | Measured from and to | What is stocked |
|---|---|---|
| Backset | Faceplate front to the middle of the euro cylinder hole | Several sizes |
| PZ | Cylinder centre to spindle centre | 48mm up to 117mm |
| Faceplate width | Edge to edge of the faceplate | 16mm up to 45mm |
| Screw centres | Fixing screw to fixing screw | Usually 215mm or 240mm |
Overall length, end to end along the strip, must match as well.
Counting Up the Locking Points
Hooks can number anywhere from none to five in the supplier's listings. Rollers appear as none, two or four; mushrooms and pinbolts as none or two; deadbolts as none up to three. Counting them takes one look at the open door with the handle up.
Gather before anyone orders: the stamp, the four measurements, the length, and a clear photo of the edge with every point thrown.
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.
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.
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
Not sure which mechanism is in your door in Bournemouth? Call HD Locksmiths Bournemouth and describe it: the stamp on the faceplate, the edge with the handle raised, and what the handle is doing. From that we can usually identify the mechanism and tell you whether the gearbox comes out separately.
Useful to have to hand: the faceplate stamp; any of the four measurements you have taken; a short description of how the handle behaves.
A technician coming out never carries a call-out charge. Calls are taken at any hour on any day of the year. Your price is set on the call from what you tell us, and any change found on site is explained to you before work goes ahead.
Describe It and Get It Identified: 01202 935089
Alternatively, call or message HD Locksmiths Bournemouth through the contact page, and ask about lock repair rather than a full replacement if that is what your door needs.
uPVC Mechanism Questions
My handle has gone floppy and the door will not lock. Is that the gearbox?
Often, the answer is yes. Those two symptoms make the gearbox a reasonable place to start looking, but they do not settle the diagnosis. Ask a qualified locksmith to assess the problem before buying parts. That keeps the choice of any replacement tied to what the examination finds.
Can I just replace the gearbox?
Yes, provided the centre case can be removed from the mechanism. This is the condition for replacing the gearbox separately, so it needs confirming before parts are chosen. If the arrangement is unclear, a qualified locksmith can identify it and explain whether gearbox-only replacement is available.
Is the cylinder part of the mechanism?
They are separate components, so no. A mention of the mechanism should not be read as a reference to the cylinder as well. If the terminology becomes confusing during a repair discussion, ask for clarification about the particular part being assessed and any replacement being proposed.
How do I find out which mechanism I have?
Take the four measurements first, then look for the maker's stamped name on the faceplate. Finish by raising the handle and counting the locking points. Keep a record of all the results, ready to share with a qualified locksmith if help identifying the mechanism is needed.
Why can my door not be left on the latch?
Automatic locking on closure is probably the reason: a slam-shut unit engages its locking points automatically when the door shuts. That makes the reported behaviour consistent with this type of mechanism. Identification is still needed before assuming that this is how the particular door is intended to work.
Why does a full strip take longer to source than a gearbox?
A full strip can take longer because its length requires matching and buying in person. Home Secure describes these strips as too long to ship. That sourcing step helps explain the difference from a gearbox: obtaining the complete strip involves an in-person purchase after a match is found.
How long should a uPVC mechanism last?
No definite lifespan can be given for this type of mechanism. A calendar-based replacement rule would suggest a certainty that is not available. If deciding whether to keep or replace a particular unit, seek an assessment from a qualified locksmith instead of choosing an arbitrary age limit.