Guide
What Is Inside a uPVC Door Lock Mechanism, and What Goes Wrong
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The Parts of a uPVC Door Lock, Named
Think of the lock in a uPVC door as three layers rather than a single object. The outer layer is the strip down the door edge with its locking points. In the middle of that strip is the gearbox, the unit that drives every point. Through both passes the cylinder that accepts the key. Calling all three "the lock" is normal, and it is also the reason quotes for an identical door can vary so much: people are often pricing different parts.
The Strip in the Door Edge and Its Locking Points
The strip is called the multipoint mechanism, and the faceplate is its exposed front. Not every door carries every type, but these are the names. Hooks rotate out and clamp the door to the frame. Rollers, which are adjustable, decide how much the door squeezes its seals. Mushrooms are made to resist prising thanks to their wide head. Pinbolts slide outwards in a straight line, as deadbolts do, though a deadbolt is typically the more substantial bolt. Keeps are the receivers on the frame that every point has to find.
The Handle, the Cylinder and the Gearbox
When the handle moves, it rotates the spindle, a square bar through the door. The follower is the square hole that bar sits in, and it belongs to the gearbox. The gearbox then pushes the whole strip at once. The euro cylinder runs through all of this but stays a separate component; a cam on it turns the gearbox when the key turns. For lock families beyond uPVC, see the full range of UK door lock types.
Four Kinds of uPVC Mechanism
Full lock, gearbox centre case on its own, overnight lock and auto slam shut: those are the four kinds of uPVC mechanism a parts supplier works with. They are the categories UPVC Spares 4 Repairs files its mechanisms under, which makes them a trade convention, not a regulation. The reason they exist is practical. They describe what is available to buy and how the door behaves on closing, and once you know one, the other usually follows.
Full Lock and Centre Case Gearbox
The full lock and the centre case sit at opposite ends of the job. One is everything in the door edge, the other only the gearbox. Your mechanism, not your preference, decides whether the smaller job is possible.
Overnight Lock, or Auto Slam Shut
On most front doors the overnight pattern is the familiar one: pull it to, lift the handle, turn the key. The slam shut kind does the locking for you as it closes, which is convenient and also the reason a door like that cannot rest on the latch.
| Type | What the door does | What a repair involves |
|---|---|---|
| Full lock | Behaves according to the strip | Taking out and matching the complete strip |
| Gearbox centre case | Carries on as normal | Changing just the centre unit, if it is removable |
| Overnight lock | Stays latched until you lift and lock | A like-for-like overnight mechanism |
| Auto slam shut | Locks as soon as it shuts | A like-for-like slam shut mechanism |
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.
What Fails, and Usually in What Order
The gearbox is the part that genuinely fails, and almost everything else on this list is wear or alignment rather than failure. It is the most common mechanism fault because the gearbox takes the load of every lift of the handle. Most mechanism failures also give notice: weeks of growing stiffness usually come first, so a door that has become hard work is telling you something.
| Part | What you notice |
|---|---|
| Gearbox | The handle goes slack, or jams part way through its travel |
| Follower and spindle | Play in the handle, a vague feel before it gives up |
| Rollers and keeps | The door needs a shove, or only some points engage; alignment, not breakage |
| Euro cylinder | The key is the problem, not the handle |
| Faceplate screws | Rare, but a loose faceplate makes everything feel worse |
This page catalogues; it does not diagnose. For that, see a door that will not lock, the door that will not open at all and a key that will not turn at all.
Keeping a Mechanism Working
Lock with the handle lifted as far as it will go, every time. That habit does more for a gearbox than anything else, because a gearbox pushed only half way still carries the load while stopping short of the end of its travel. Four further points complete the routine.
Edge, not keyhole
the hooks, rollers and bolts are what move, so lubricate them with a product designed for locks.
Clear keeps and threshold
obstructions there make each locking point push harder than it should.
Early stiffness
respond as soon as it shows, since it is the notice that comes before most failures.
Light handles
heavy items hung on the handle keep the spindle and gearbox under constant load.
Expect a door to behave a little differently between the seasons too. And keep expectations honest: looking after a mechanism postpones the day the gearbox fails, but it cannot stop that day coming, because the part is built to wear.
Gearbox Alone, or the Whole Strip?
Whether the gearbox can be changed on its own is decided by the mechanism, not by the person quoting. Three things settle it, in this order:
A removable centre case.
Where the mechanism is built with one, a gearbox swap leaves every hook, roller and the faceplate exactly where they are.
No removable centre case.
Here a gearbox fault means a full strip, and that strip has to match on all four measurements and every locking point, the specifications UPVC Spares 4 Repairs filters its stock by.
Length.
Home Secure's changeover guide states that full multipoint locks are too long to ship, so they have to be matched and bought in person, while gearboxes can be posted.
That third point explains most of what puzzles people about this job. A gearbox can often be sourced quickly. A full strip usually means somebody standing at the door with the old one in their hand, matching it part for part. Nobody chooses the bigger job; the door's mechanism chooses it.
Get Your Mechanism Identified
Start With a Description
Give Big Locksmiths Boston a call and talk us through the door: what is stamped on the faceplate, what the edge looks like with the handle lifted, and how the handle feels. In most cases that is enough for us to name the mechanism and say whether the gearbox can be replaced alone.
Worth Having Ready
The stamp, the four measurements if you have managed to take them, and a note of what the handle is doing will all shorten the conversation.
There is never a charge for a technician attending. Lines are open day and night, every day including bank holidays. We price the job on the call from your description, and should anything at the door change that, you are told before the work starts.
Describe It and Get It Identified: 01205 601044
Or use the contact page to call or message Big Locksmiths Boston.
uPVC Mechanism Questions
My handle has gone floppy and the door will not lock. Is that the gearbox?
Yes, the gearbox is a strong possibility here. A handle that has become floppy, together with an inability to lock, gives a reason to suspect it. The next step is a check by a qualified locksmith, so any decision about replacement follows an assessment of the actual mechanism.
Can I just replace the gearbox?
Only a mechanism with a detachable centre case allows that approach. Start by confirming this detail rather than assuming the gearbox can be changed separately. Once the mechanism has been identified, a qualified locksmith can establish whether it meets that condition and advise on the replacement.
Is the cylinder part of the mechanism?
No: a cylinder and a mechanism are different components. That distinction matters when interpreting repair advice. Rather than using the terms interchangeably, ask for the relevant part to be named clearly. This keeps a discussion about a possible replacement focused on the component being assessed.
How do I find out which mechanism I have?
Gather the four measurements alongside the maker's name and locking-point count. Find the manufacturer stamped on the faceplate, and do the counting with the handle raised. These are the details to have ready when identifying the fitted mechanism, so record them in one place.
Why can my door not be left on the latch?
The door may use a slam-shut arrangement that automatically throws the locking points on closing. If so, that explains the difficulty leaving it on the latch. This is a probable explanation, not a confirmed identification; a qualified locksmith can establish which mechanism is actually fitted.
Why does a full strip take longer to source than a gearbox?
The full strip needs to be matched and obtained in person because of its length. That is the explanation from Home Secure, which describes it as too long for shipment. Completing the match and purchase this way helps explain why sourcing can take longer than for a gearbox.
How long should a uPVC mechanism last?
The lifespan of a uPVC mechanism is not a fixed period. That means there is no definite age-based deadline to use when planning replacement. Have the actual mechanism assessed by a qualified locksmith if its future is in question, rather than working from a supposed expiry point.