Summary

uPVC and composite doors almost universally use a multipoint locking system: a single handle or key action drives a central gearbox that simultaneously throws hook bolts, rollers, and shootbolts into keeps in the frame. It's a strong system when everything is aligned and undamaged, but it has several distinct failure points, and tradespeople — and customers — routinely blame the wrong one. A customer who says "the lock's broken" might have a snapped cylinder, a worn gearbox, a dropped door binding on its hinges, or misaligned keeps. Each has a different fix, and only one of them actually requires buying a new mechanism.

This matters commercially as well as technically. A worn gearbox at £30–£70 is a very different job from a full door replacement at £600+. Misdiagnosing a hinge-alignment issue as a lock fault means the customer pays for a new mechanism that doesn't fix the symptom, and the tradesperson has to go back. Getting the diagnosis right — and then measuring correctly if a mechanism genuinely needs replacing — is the difference between a 45-minute job done once and a return visit with an apologetic customer.

The other recurring issue is security. Since lock-snapping became the dominant method of forced entry to UK homes with uPVC and composite doors, any mechanism replacement or cylinder swap is also an opportunity — and increasingly an expectation — to bring the door up to TS007 3-star anti-snap standard.

Key Facts

Quick Reference Table

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Symptom Likely Component Typical Fix Rough Cost Guide
Key turns but door doesn't lock/unlock Snapped or worn cylinder cam Replace euro cylinder (matched size) £20–£70 + fitting
Handle lifts but hooks don't engage Worn gearbox Replace gearbox/centre case £30–£120 + fitting
Handle feels loose or "floppy" Worn follower/spindle or gearbox internals Replace spindle and/or gearbox £15–£90 + fitting
Door hard to close, needs "lifting" or slamming Dropped door / worn hinges Adjust hinges, check keeps £0–£60 (adjustment only)
Key sticks or is hard to turn Misaligned keeps, dirty/worn cylinder, or door sag Adjust keeps/hinges first; lubricate; replace cylinder if worn £0–£70
Visible snap on outside of cylinder Attempted forced entry (lock snapping) Full cylinder replacement, TS007 3-star £30–£70 + fitting
Whole mechanism seized or corroded Gearbox internals corroded/frozen Full mechanism replacement, matched to brand £60–£250 + fitting
Mechanism obsolete, no direct replacement available End-of-life gearbox Aftermarket compatible case, or door/frame replacement £150–£600+
Door won't latch even with handle down Latch spring failure or keep misalignment Latch/gearbox replacement or keep adjustment £30–£100 + fitting
Hooks visible but not seating fully in keep Frame distortion or keep wear Adjust/replace keeps; assess frame squareness £20–£150

Detailed Guidance

Anatomy of the mechanism

A uPVC multipoint lock is routed into a slot in the door edge and secured by the faceplate (the visible strip on the door edge, held by 4–8 screws). Inside the case sits the gearbox — a compact assembly of gears and cams that, when the handle is lifted, drives the hooks and shootbolts outward simultaneously, and when lowered, retracts them. The euro cylinder passes through a hole in the door face and into the gearbox, where its cam engages the deadlocking function so the hooks stay thrown even with the handle down, and can only be released by turning the key.

The latch is a separate spring-loaded bolt at the same height as the handle, and operates independently — this is why a door can "hold shut" even with the multipoint fully unlocked; customers sometimes describe this as "the lock still working" when in fact only the latch is functioning and the deadlocking hooks are not engaging at all.

Measuring for a replacement mechanism

Four measurements are required before ordering, and getting any one wrong means the part won't fit:

  1. Backset — edge of door to centre of spindle hole. Measure directly; don't assume from door width.
  2. PZ (centre-to-centre) — centre of spindle to centre of cylinder hole, measured vertically. This is the figure most often quoted wrong because installers measure to the edge of the hole rather than the centre.
  3. Case length — the overall length of the gearbox faceplate, end to end. Measure the existing faceplate, not the door.
  4. Door thickness — affects the cylinder length required (cylinders are specified as two half-lengths either side of the centre line, e.g. 35/45) and the correct spindle/follower length.

Always measure the existing mechanism directly rather than working from door dimensions alone — even doors that look identical externally can have different case configurations depending on the installer and the era of installation.

Common failure modes and how to tell them apart

Worn gearbox — the handle lifts and lowers with normal resistance, but the hooks either don't fully engage, or the key turns without the deadlock actually catching. Often accompanied by a gritty or notchy feel in the handle action as gears wear.

Snapped or worn cylinder — the key turns freely (sometimes 360° with no resistance) without operating the lock at all, or won't turn past a certain point. Visually inspect the exposed end of the cylinder on the outside face for a clean snap line — a strong indicator of an attempted lock-snapping attack rather than simple wear.

Misaligned keeps — the hooks are visibly present and moving when the handle is operated, but don't seat into the keep, or only partially engage. Often caused by the door dropping slightly over time, or the keeps being knocked out of position. Adjustable keeps can usually be shimmed or repositioned rather than replaced.

Sagging door / worn hinges — check this first, always. Open the door and lift the handle end — if there's visible movement (more than a couple of millimetres), the hinges have worn or the door has dropped on its frame. This misaligns everything downstream: the hooks no longer line up cleanly with the keeps, and the whole mechanism can feel "stiff" or "binding" even though the gearbox itself is fine. Hinge adjustment (most uPVC hinges have adjustment screws for both vertical and lateral movement) should always be tried before any part is replaced.

Euro cylinder security and anti-snap ratings

Since lock snapping became the dominant forced-entry method for uPVC and composite doors, any cylinder replacement is also a security upgrade opportunity. TS007:2014+A1:2015 grades cylinders and door furniture from 1-star to 3-star for resistance to snapping, picking, drilling, and bumping. A cylinder rated 3-star on its own meets the insurance-recognised standard; alternatively a 1-star cylinder combined with 2-star rated door furniture (handles/escutcheons that protect the cylinder face) achieves the same combined rating. Secured by Design, the UK police-endorsed accreditation scheme, recognises both routes. See security hardware for full detail on cylinder grading and door furniture options — the same principles apply directly when replacing a cylinder as part of a mechanism repair.

A door with a sound, well-fitted multipoint mechanism but a basic non-rated cylinder is still highly vulnerable — the hook bolts themselves are rarely the weak point in an attack; the cylinder almost always is.

Diagnostic approach: cylinder, gearbox, or door alignment

The most common misdiagnosis on-site is jumping straight to "the lock's gone" and replacing the gearbox, when the actual fault is either the cylinder or the door alignment. A practical order of checks:

Door won't lock/unlock properly
        |
        v
Does the door sit square and flush in the frame?
(lift handle end, check for drop/binding)
   |                              |
  NO                             YES
   |                              |
Adjust hinges/keeps first    Operate handle only (cylinder removed
Re-test before touching       or door open) — do hooks/shootbolts
lock mechanism                move freely and fully?
                                 |                    |
                                NO                   YES
                                 |                    |
                          Gearbox worn —        Test cylinder alone:
                          replace mechanism      does key turn cam smoothly?
                                                      |            |
                                                     NO           YES
                                                      |            |
                                              Replace cylinder   Fault likely
                                              (TS007 3-star)     in keeps/frame
                                                                 alignment

Sourcing and fitting the correct replacement mechanism

Multipoint mechanisms are brand- and model-specific. GU, Winkhaus, Mila, ERA, Yale, and Fuhr are among the major UK suppliers, and two gearboxes with an identical case length are frequently not interchangeable — hook spacing, fixing hole positions, and faceplate profile (flat, radius, or grooved) all vary between manufacturers. Where possible, identify the brand from markings stamped on the faceplate or gearbox body, and order a like-for-like replacement rather than a generic "universal" case, which often requires re-routing the door edge and can weaken the fit.

When to recommend a full door or frame replacement instead

A mechanism swap isn't always the right answer. Recommend a full door (or door and frame) replacement rather than a like-for-like mechanism when:

In these cases, be upfront with the customer that a mechanism swap is a temporary fix that is likely to fail again, rather than quoting for a repeat visit later.

Frequently Asked Questions

The customer says the lock is "stiff" but still works — is a replacement urgent?

Not necessarily, but it should be checked. A stiff handle is often the first sign of a dropping door or a gearbox starting to wear, well before it fails completely. Check hinge alignment and keep position first — a five-minute adjustment can resolve stiffness without any parts needing replacement. If stiffness persists after alignment is corrected, the gearbox is likely wearing and should be flagged for replacement before it fails outright (often when the customer is locked out).

Can I fit a cheaper "universal" mechanism instead of the original brand?

Generic mechanisms exist and can work for standard case lengths, but hook spacing and fixing positions frequently don't match the original routing in the door edge, requiring modification to the door itself. For a clean, reliable fit — and to avoid comebacks — sourcing the correct brand and model is almost always worth the extra cost and lead time, particularly on doors still within a reasonable service life.

Is it worth upgrading the cylinder every time a mechanism is replaced?

Generally yes, if the existing cylinder isn't already TS007 3-star rated. Since the mechanism is already being worked on and the door is open, the marginal labour cost of also fitting a rated cylinder is small compared to a separate future visit — and it directly addresses the highest-risk component for forced entry. Always confirm sizing (see measuring section) rather than assuming the same cylinder length as the one removed, especially if the door has been re-painted or refurbished with different furniture.

How do I know if a gearbox is genuinely worn versus just needing lubrication?

Try a silicone-based lubricant (not oil-based, which attracts grit) on the gearbox and keeps first — a mechanism that has simply dried out can often be restored to smooth operation this way. If the handle action remains gritty, notchy, or the hooks still fail to fully engage after lubrication and a keep check, the gearbox internals are genuinely worn and replacement is the appropriate next step.

What's the difference between the lock mechanism and the door handle set?

The handle set (external and internal handles, spindle, and any escutcheons) is separate from the gearbox and cylinder, though all three interact. A worn or damaged handle set can cause similar symptoms to a gearbox fault — a loose or "floppy" feel is often a worn spindle/follower rather than the gearbox itself. Replacing the handle set alone is cheaper and should be ruled out before assuming the gearbox needs replacing.

Regulations & Standards