uPVC Door Lock MechanismGearbox Repair & Replacement
The gearbox inside your multi-point strip is the part doing the work. When it fails, the hooks stop deploying, the handle goes slack or the key jams mid-turn. We identify, match and swap every major brand of gearbox same-visit across every South London postcode — Yale, ERA, Mila, GU Ferco, Fullex, Avocet, Winkhaus.
Fixed prices · No VAT · No call-out fee
The cassette at the heart of every uPVC door — and what happens when it gives up.
Look at the open edge of your uPVC door and you will see a long metal strip running from near the top to near the bottom. That is the multi-point strip. Bolted to the centre of that strip, hidden behind the handle backplate, is the gearbox — a sealed steel cassette about the size of a small paperback. This is the mechanism. Every lift of the handle, every turn of the key, every hook that deploys and every roller that compresses, is driven by the gears, springs and cams inside this single cassette.
When homeowners say "my uPVC door lock has broken", nine times out of ten they mean the gearbox has failed. The multi-point strip itself rarely fails — the shoot bars, hooks and rollers are simple steel parts that wear slowly and are almost always reusable. The euro cylinder is a separate part that can be replaced independently in 15 minutes. The mechanism is the middle layer: where precision-machined internal gears take handle rotation and translate it into vertical drive, and where the cylinder cam either permits or blocks that drive. When gears wear or a spring snaps, the door still looks fine from the outside — but lifting the handle either does nothing or does the wrong thing.
What this page covers
A full technical breakdown of what is inside the gearbox, the four common mechanism families found on UK doors, how to identify which one is on your door, the failure modes we see weekly, and what a like-for-like replacement costs. If the problem is clearly the cylinder and not the mechanism, our uPVC door locks page covers that. For the service itself see uPVC door lock repair. For multi-point as a whole system see multi-point locking system.
If you know you need the job booked, one photo of the faceplate stamp on WhatsApp and we will identify the gearbox brand, case size and backset within minutes — no visit required just to quote. Coverage details on our areas we cover page. For background on the team, the about us page and our contact page are the quickest routes to get the work on the calendar. For wider reading see our locksmith advice blog.
Nine parts inside a uPVC door lock mechanism — and which one usually fails first.
The gearbox looks like a simple black or silver rectangle. Inside are nine interlocking precision parts that have to work in exact sequence. Understanding which part does what turns a mysterious "door won't lock" callout into a 30-second diagnosis.
Follower (Spindle Hub)
The square socket that the handle's spindle passes through. Rotates 45° when you lift the handle. The first point of wear — a sloppy follower is why handles droop.
Return Spring
Pulls the follower back to horizontal after you release the handle. A weak or broken spring is why the handle drops and refuses to spring back. Single most replaced internal part.
Cam Plate
The vertical steel plate connected to the follower by a rack-and-pinion gear. Converts rotation into the vertical drive that moves the shoot bars up and down.
Transmission Gears
Precision-cut gears that step the motion between follower, cam and shoot-bar couplings. Hardened tool-steel on good brands; pressed-steel on budget imports that fail quicker.
Cylinder Cam Slot
The slot that receives the cam on the back of the euro cylinder. When the key turns, the cam drops into this slot and locks the cam plate. No key turn = no movement possible.
Deadbolt Drive
On mechanisms with a centre deadbolt, a separate linkage drives a square bolt into the frame at handle height. Keyed operation only. Adds a dedicated hardened locking point.
Shoot-Bar Couplings
Small hardened pins that link the cam plate to the shoot bars above and below the gearbox. When these shear, the handle still lifts but the hooks never deploy.
Anti-Slam Pawl
Only on "anti-slam" gearboxes. A small pawl that prevents the deadbolt firing if the door is slammed open. Protects against wind-gust deadbolt engagement that would smash into the frame.
Housing & Faceplate
The metal housing sealing the cassette and the silver/brass faceplate screwed to the door edge. Brand stamp and reference number are printed here — this is what we photograph to identify replacements.
Four gearbox patterns covering 99% of UK uPVC doors.
Every uPVC mechanism falls into one of four mechanical families. Which one you have decides the replacement part number, the spindle geometry and whether a like-for-like swap is possible. Identify yours before you order a gearbox on Amazon — the wrong family cannot be retrofitted without reshaping the door pocket.
Single-Spindle Gearbox
One spindle runs through the follower. Inside and outside handles operate as a pair — lifting either one deploys every locking point. The default layout on standard residential uPVC front and back doors.
- PZ: 92mm euro cylinder centre to spindle
- Backset: 35mm typical
- Common brands: Yale, ERA, Mila, Fullex
- Cost: gearbox from £350 fitted
Split-Spindle Gearbox
Two independent spindles. The inside handle always retracts the deadbolt (no key required from inside). The outside handle needs the cylinder turned first. Required for fire-escape compliance in HMOs and tenanted houses.
- Safety: key-free egress from inside
- Compliance: HMO, flat entrance doors
- Common brands: Winkhaus, GU Ferco, Fuhr
- Cost: WhatsApp for quote
85 PZ Narrow-Stile Gearbox
Built for slim-stile composite and aluminium doors where there is not enough door width for the standard 92mm PZ layout. Found on narrow sidelight doors and some patio/French door configurations.
- PZ: 85mm (not 92mm)
- Cylinder needs shorter total length
- Common brands: Yale Platinum, Winkhaus
- Specialist source: 24-hr lead time
Anti-Slam & Auto-Engaging Gearboxes
Premium gearboxes that either resist deadbolt engagement on door slam (anti-slam) or automatically deploy hooks when the door closes without lifting the handle (auto-engage). Standard on high-end composite doors.
- Anti-slam: protects deadbolt from gusts
- Auto-engage: one-action closing
- Common brands: Fuhr, Winkhaus AV2, Avocet
- Cost: WhatsApp for quote
Five measurements that tell us exactly which gearbox your door needs.
Get these five numbers off your door and we can match the replacement cassette in minutes — no fitting visit required before we confirm the quote. Every measurement is taken with a tape from the door edge; no tools required.
Backset
Distance from the faceplate to the centre of the spindle square. Usually 35mm on standard doors.
PZ Measurement
Centre-to-centre distance between the keyhole and the spindle. 92mm standard, 85mm narrow-stile.
Faceplate Height
Overall gearbox case height measured along the door edge. 244mm, 270mm and 300mm are common.
Faceplate Width
Width of the visible faceplate strip. 16mm is standard; some older Lockmaster and Maco are 20mm.
Brand Stamp
Every gearbox has a manufacturer stamp and reference code on the faceplate. Photograph this first.
Easier — just send a photo.
Open the door, take one photo of the faceplate (the visible brass or silver strip on the door edge) with the brand stamp and reference clearly readable. One more photo of the keyhole backplate. Send both on WhatsApp and we identify the gearbox family, brand, exact case size and backset before anyone leaves the workshop.
Send Photo on WhatsApp- BrandYale / ERA / Mila / Fullex
- FamilySingle / Split / Anti-slam
- PZ92mm or 85mm
- Backset35mm / 40mm / 45mm
- PriceExact quote, fixed
Six mechanism failure patterns — and which internal part is behind each one.
Read the symptoms before you call. Telling us which of these six patterns you see knocks 20 minutes off the visit because we can arrive with the right part in hand. The fault is almost always one specific internal component — not the whole mechanism.
Handle lifts but nothing deploys
The handle travels the full 45° but the hooks and rollers stay inside the door edge. The tiny hardened pins linking the cam plate to the shoot bars have sheared, disconnecting the drive from the bars.
Handle droops & will not return
The handle sags down instead of springing back horizontal after you release it. The coil or leaf spring that returns the follower has lost tension or broken. Very common after 10–15 years.
Handle lifts part-way and jams
The handle stops at around 20° and refuses to go further. A tooth on the internal transmission gear has chipped or bent, blocking further rotation. Forcing it accelerates the damage.
Key turns but handle will not lift
The key rotates freely but the handle feels locked rigid. The cam on the back of the cylinder is stuck inside the cam slot of the gearbox, blocking cam plate movement. Often a misaligned cylinder.
Grinding noise when lifting
Lifting the handle produces a grinding or metallic rasp. The follower's square socket has worn to a rounded shape — the spindle rotates inside the worn socket rather than driving the gears. Common on 15+ year doors.
Locked in — cannot exit from inside
Key turns from the inside but the hooks will not retract. Internal spindle has sheared, or the cam plate has seized from corrosion or foreign material. Emergency — we prioritise these callouts.
The decision tree we walk every customer through on the phone.
The gearbox is a sealed cassette — internal gears and springs are not user-serviceable. "Repair" in practice means swapping the cassette; the rest of the multi-point strip is almost always reusable. Here is how we decide what gets swapped and what stays.
Door has dropped on hinges — hooks miss keeps
The gearbox is fine. The problem is the hooks are reaching the frame below the strike holes. Hinge adjustment plus keep realignment solves it. No gearbox purchase.
Typical cost: £120–150Handle or spring fault — shoot bars still work
The faulty part is inside the cassette. Shoot bars on the multi-point strip test clean. Swap the gearbox cassette, keep the strip, keep the keeps. Most common repair we do.
Typical cost: from £350Shoot bars bent or corroded
Vertical bars above or below the gearbox have bent from forcing, or corroded from water ingress. Full multi-point strip replaces the entire run: gearbox, shoot bars, couplings, and new brand-matched keeps if required.
Typical cost: from £450Handle spring cassette worn
Handle droops but gearbox is fine inside. Replacing just the handle with a new spring cassette restores full operation. Done at the same visit if diagnosed in advance.
Typical cost: from £150Door frame corroded or shoot rails bent in combination
If both the gearbox and the frame keeps are beyond service, the economics shift. We will recommend a new door at this point rather than replace parts that are already near end-of-life. Honest advice over upsell.
Recommendation: replace doorWorking but you want 3-star security
If the mechanism is still operating well but your euro cylinder is a basic unit, upgrade the cylinder only. The mechanism does not need to be touched to pass insurance wording.
Typical cost: from £200A replaced gearbox adds 15 years of service life — a new door costs twelve times as much.
We see homeowners quoted £1,200+ for a new uPVC door when the actual fault is a £350 gearbox. On doors less than 12 years old, with a sound frame, a gearbox swap brings the door back to factory operation. We will not fit a gearbox into a door that does not warrant it — the free mechanism audit tells you honestly which it is.
Four services that cover every mechanism job we take.
Each service below is what we show up for. Fixed pricing. Same-visit completion where possible. Brand-matched gearbox stock on the van to avoid a return trip.
Gearbox Diagnosis & Repair
Identify the failed internal component from symptoms, confirm brand and reference, replace the gearbox cassette, test operation from both sides.
Full Multi-Point Strip
Replacement of the entire run: new gearbox, new shoot bars, new couplings and realigned keeps. For doors with bent or corroded strip hardware.
Split-Spindle Upgrade
Swap a single-spindle gearbox for a split-spindle version to meet HMO fire-escape compliance. Legal requirement for many tenanted and shared properties.
Emergency Mechanism Open-Up
Door locked with a failed gearbox, locked-in scenario, key stuck or handle jammed. 24/7 emergency entry, non-destructive where possible, new mechanism fitted same visit.
Three steps between your photo and a working door.
Send faceplate photo
Photograph the brass or silver faceplate on the door edge with the brand stamp clearly readable. One photo on WhatsApp. We identify the gearbox family, PZ, backset and brand before anyone travels.
≈ 5 minutes30-minute arrival, gearbox in hand
Uniformed DBS-checked engineer arrives with the matching Yale / ERA / Mila / GU Ferco / Fullex / Avocet gearbox and the tools to swap the cassette cleanly. ID shown at the door first.
CR / SW / SE / SM / BRFitted, tested, guaranteed
Cassette swapped in the live multi-point strip, keeps realigned if needed, door tested 10 times from both sides. Written invoice with 12-month workmanship guarantee. Card or cash paid on completion.
Typically 45 minSpecialist gearbox diagnosis — no guesswork, no second visits.
Half the market arrives on a mechanism callout without the right part. We don't. Photo first, brand matched, fitted same visit, guaranteed.
Photo-First Diagnosis
We identify the gearbox brand, family and exact reference before dispatch — no "we’ll see when we get there".
8+ Brand Gearbox Van Stock
Yale, ERA, Mila, GU Ferco, Fullex, Avocet, Winkhaus, Lockmaster. Fitted from van stock, no wholesaler trip.
Fixed Price, No VAT
Quote given after photo. Stays quote. If the job reveals hidden damage, you agree any extra before we progress.
30-Minute Typical Response
Most SW, SE, CR, SM, BR callouts on site within half an hour. 24/7 cover including nights and bank holidays.
No Drill-First Policy
We never drill a gearbox out as the first response. Non-destructive entry where possible. You save the cost of an avoidable replacement.
12-Month Guarantee
Cassette, bar couplings, handle alignment — all covered. Failure for a reason we caused — we come back free.
DBS-Checked Engineers
Every engineer background-checked, uniformed, ID-carded. Shown at the door before we cross your threshold.
Written Invoice & Spec Sheet
Paper trail for your insurer, landlord or property manager. Brand and reference of fitted gearbox recorded.
Free Mechanism Audit
If you want to know whether your current gearbox is near end-of-life before a failure, book a free audit. No pressure, no upsell.
Card & Cash Accepted
Paid on completion, never upfront. No deposits, no holding fees.
Mechanism pricing — no call-out fee, no VAT on top.
Every figure below is the starting price for a standard South London residential job during business hours. After-midnight and bank-holiday rates are quoted separately before dispatch.
Mechanism Audit
- Identify brand & reference
- Spot failure mode
- Fixed quote back
Gearbox Replacement
- Brand-matched cassette
- Shoot bars retained
- Keep realignment
- 12-month guarantee
Split-Spindle Upgrade
- Fire-escape compliant
- Compliance certificate
- Winkhaus / GU Ferco stock
Multi-Point Replacement
- New gearbox + shoot bars
- Full frame realign
- Brand-new keeps
What customers say after a mechanism callout.
Another locksmith quoted me for a complete new door. Sent a photo here instead and was told the gearbox was the only problem. Arrived the next morning with the matching Fullex cassette, fitted it, the door works like new. Saved me about £800.
Landlord had a HMO inspection coming up, needed split-spindle gearboxes on four flat entrance doors. They sourced matched Winkhaus replacements, fitted all four in a morning, gave me a compliance letter on the spot that the council accepted.
Handle had been drooping for months. I'd been told it was "just age" by two different handymen. Sent a photo here, got a clear explanation that the spring cassette inside the gearbox had gone, booked in and fixed. Clean work, no mess.
Every question we get asked about a failed uPVC door lock mechanism.
What is a uPVC door lock mechanism?
The mechanism is the gearbox — the sealed cassette housed in the centre of the multi-point strip that drives every locking point. When you lift the handle, a follower inside the gearbox translates rotation into vertical movement, pushing the hooks and rollers out into the frame. When you turn the key, the cylinder cam locks the gearbox so nothing can move until the key turns back. See our uPVC door locks page for the whole door system.
What is the difference between the gearbox and the cylinder?
The cylinder is the keyed barrel. The gearbox is the mechanism that moves the hooks, rollers and shootbolts. The cylinder tells the gearbox whether to allow handle operation — but the gearbox does the mechanical work. Either can fail independently; correct diagnosis decides what needs replacing. Read more on the euro cylinder lock page.
How do I know if my uPVC door lock mechanism has failed?
The handle lifts freely but no hooks deploy. The key turns but the handle will not lift. A grinding noise when lifting. The door locks but will not unlock. Or the handle lifts part-way and sticks. Each of these points at a specific failed part inside the gearbox — the follower, the spring cassette, the cam plate or one of the transmission gears. The uPVC door lock repair page walks through diagnostics.
Can a uPVC door lock mechanism be repaired?
The gearbox itself is a sealed cassette — individual internal gears and springs are not user-serviceable. Repair in practice means replacing the whole cassette. The rest of the multi-point strip (shoot bars, keeps) is usually reusable, which is why a gearbox-only repair from £350 is cheaper than a full multi-point strip replacement from £450.
How long does a uPVC lock mechanism replacement take?
A straightforward like-for-like gearbox swap takes 30 to 45 minutes on site. A full multi-point strip replacement takes 60 to 90 minutes. If the door has dropped on its hinges we realign the keeps at the same visit — factor in another 20 minutes. Every job is completed in one attendance.
Do all uPVC door lock mechanisms fit all doors?
No — mechanisms are brand-specific and within each brand there are multiple variants. The gearbox has to match the spindle height (the distance from the faceplate to the follower square), the backset (the depth of the gearbox inside the strip) and the PZ measurement (92mm or 85mm euro cylinder distance). We identify yours from a photo of the faceplate stamp before ordering.
What is a split-spindle gearbox?
A split-spindle gearbox uses two independent spindles — one for the inside handle and one for the outside handle. The outside handle cannot retract the deadbolt without the key, but the inside handle can always open the door in an emergency. Standard single-spindle gearboxes operate both handles together. Split-spindle is required for HMO fire-escape compliance.
How much does a uPVC door lock mechanism cost to replace in South London?
A gearbox-only replacement starts from £350 including the new cassette, labour and keep realignment. A full multi-point strip replacement starts from £450. Prices depend on brand availability — specialist GU Ferco, Winkhaus and Fuhr mechanisms may need a 24-hour source lead-time at no additional callout fee.
How long should a new uPVC door gearbox last?
A good-quality OEM-matched gearbox will last 12 to 15 years of normal residential use. Budget aftermarket cassettes tend to fail in 5 to 8 years. We only fit OEM-matched or OEM-equivalent gearboxes for this reason — saving £40 on a cheaper cassette is a false economy when you factor in the callout to replace it early.
Related lock types, repairs and location coverage.
If the mechanism is only one part of a bigger uPVC security review you're working on, the links below cover every adjacent topic, service and South London location we attend.
Explore the site
Related locks & problems
South London locations we cover
uPVC door mechanism fixed today across South London.
Photo-first diagnosis, brand-matched gearbox stock on every van, fixed prices from £350. No call-out fee, no VAT, 12-month workmanship guarantee. One photo on WhatsApp and the quote lands back before the kettle boils.
24/7 emergency locksmith serving South London since 2015 — trusted by homeowners, landlords and businesses across Sutton, Mitcham, Wallington, Cheam, Purley and surrounding areas.