A uPVC door locking mechanism is the long metal strip on the edge of the door that throws hooks, bolts or rollers into the frame when you lift the handle. Its main parts are the euro cylinder (where the key goes), the centre gearbox (which drives everything), the multipoint strip and faceplate, the keeps on the frame, and the handles and spindle. When something fails, you can usually replace that one part rather than the whole door.
This guide explains what each part does, how to work out which one has failed, the measurements you need before ordering a uPVC door lock mechanism replacement, and how to choose parts that improve security rather than just restore it.
How a uPVC multipoint lock works
When you lift the handle, the gearbox in the middle of the door converts that movement into a coordinated throw of several locking points along the strip. Each one engages a keep fixed to the frame. Turning the key in the cylinder then deadlocks the mechanism, so the handle cannot simply be pushed back down.
Spreading the locking across several points makes the door harder to force and pulls it tight against its seals. The catch is alignment. If the door has dropped or the keeps have moved, you will feel the handle getting stiff, and that extra friction wears out the gearbox. Most “broken lock” call-outs start with a door that has been slightly out of line for months.
The main parts, explained
| Part | What it does | Common symptoms when it fails |
|---|---|---|
| Euro cylinder | Takes the key; its cam drives the gearbox to lock and unlock | Key stiff or will not turn; key turns but nothing happens |
| Centre gearbox (centre case) | Houses the latch and usually a deadbolt; converts handle and key movement into movement along the strip | Floppy handle, latch not returning, lock will not engage |
| Multipoint strip and faceplate | The full-length rail that carries the locking points | Stiff all the way along, even with the door open; bent faceplate |
| Locking points | Hooks, bolts, rollers or mushroom cams that engage the frame | Rub marks, scraping, door not pulling tight |
| Keeps (strikers) | Receivers on the frame that the locking points engage | Handle stiff only when the door is closed |
| Handles and spindle | Transmit your hand movement to the gearbox | Droopy lever, loose handle, rounded spindle |
The centre gearbox
The gearbox takes most of the strain, which is why it is one of the most commonly replaced parts. To identify a replacement, note any brand name or code stamped on the faceplate, then measure the backset and PZ (see below), the case height and depth, and the position of the fixing screws. Specialist suppliers such as Lock Shop Direct warn not to order on brand or backset alone, because many gearboxes look similar.
Locking points
- Hooks grab the keep and resist the door being levered or lifted.
- Deadbolts and pin bolts are straight throws that add rigidity.
- Rollers and mushroom cams pull the door into its seals; some designs also resist lifting.
Euro cylinder
The cylinder is the easiest part to change and the most important security upgrade. Police forces recommend a cylinder rated TS 007 3 star, or a 1 star cylinder combined with 2 star security handles, according to West Yorkshire Police. Cylinders tested to Sold Secure SS312 Diamond are also tested to a high standard, and some carry both ratings. TS 007 testing covers attacks including snapping, drilling, picking and bumping, as explained in this TS 007 guide.
When measuring, take the retaining screw out and measure from the screw hole to each end of the cylinder separately; many cylinders are offset. The outside end should sit close to flush with the handle. West Yorkshire Police’s door and window advice says it should not protrude more than 3mm beyond the door furniture.
Handles and spindle
A single spindle is one square bar running through both handles, so either handle opens the door. A split spindle is in two halves: the outside lever lifts to lock but will not open the door, so you need the key to get in from outside. A lever/pad set has a fixed pad outside and works the same way from outside, as this single vs split spindle guide explains. Before switching to a split spindle or lever/pad set, check that turning the key withdraws the latch on your lock; if it does not, you could lock yourself out.
Most uPVC handles use an 8mm square spindle, but measure yours to confirm. A worn, rounded spindle can make a handle feel loose and imprecise, and is a cheap part to replace. Sprung handles help if the gearbox’s own springs are tired and the lever droops.
The measurements you must get right
Work with the door open, photograph each step and measure to centres, not edges.
- Backset: from the front of the faceplate to the centre of the cylinder or spindle.
- PZ (centres): from the centre of the spindle to the centre of the cylinder. According to Door Furniture Direct, 92mm is the most common on modern multipoint locks, 68mm is often found on older doors and 70mm is less common.
- Handle screw centres: centre of the top fixing screw to centre of the bottom one, so the new handles cover the old holes.
- Gearbox size: case height and depth, and the fixing screw positions.
- Faceplate: width, profile (flat or U-rail), end shape and fixing-hole positions.
- Strip length and point positions: essential if you are replacing the full mechanism.
- Spindle: size and whether it is single or split.
- Cylinder length: from the retaining screw to each end.
Gearbox only, or the full mechanism?
Replace just the gearbox when the strip moves freely with the door open, the locking points are not worn, the faceplate is straight and you can find a matching gearbox. It is usually the quicker, less disruptive job.
Replace the full mechanism when several locking points are seized or worn, the faceplate is bent, the screw holes are chewed, or no matching gearbox is available. You may also need to adjust or replace the keeps so the new points line up.
Either way, fix the door’s alignment first. A new part fitted to a dropped door will start wearing straight away.
Quick diagnostics
Start with the door-open test: unlock the door, open it and lift the handle. If it is smooth when open but stiff when closed, the problem is alignment, not the lock. If it is stiff even when open, look at the gearbox or the strip.
- Floppy handle: take the handles off, put the spindle back in and turn it by hand. If the latch springs back firmly, the handle springs are tired, so fit sprung handles. If the latch feels lifeless, the gearbox springs have probably failed. Check the spindle for rounding too.
- Handle stiff only when the door is shut: look for rub marks on the hooks and keeps, then adjust the keeps or hinges in small steps.
- Key turns but nothing happens: with the door open, watch the bolt while turning the key. If nothing moves, the cylinder cam may not be engaging the gearbox, or the gearbox has failed internally. This is a good point to call a locksmith.
Maintenance that prevents failures
- Adjust the hinges so the locking points enter the keeps cleanly.
- Reposition keeps that show scraping marks.
- Lubricate the moving parts of the mechanism lightly, using a product the lock maker recommends, and keep grit out of the keeps.
- Tighten handle bolts, the cylinder retaining screw and hinge screws.
- Lift the handle fully before turning the key, and never force a stiff lock: find the cause instead.
Our guide to the top weak points of uPVC doors covers hinges, glazing and letterboxes as well as locks.
DIY or locksmith?
Swapping a cylinder or handles is a manageable DIY job for most people with a screwdriver and accurate measurements. Replacing a gearbox or full strip is more involved: you need the door wedged open, the latch taped back so it cannot click shut, and confidence that the replacement matches. If you are unsure, a Master Locksmiths Association member can identify, supply and fit the right parts, as West Yorkshire Police suggest. Ask for an itemised quote covering parts, labour and any adjustment.
When buying, choose suppliers that show full dimension drawings and will match your lock from photos and measurements.
Security upgrades while you are at it
- Cylinder: TS 007 3 star, or Sold Secure Diamond. See our anti-lock snapping guide.
- Handles: TS 007 2 star security handles, which can be paired with a 1 star cylinder.
- Spindle: the Security Spindle – HOLDOOR is an 8mm boxed steel spindle, 138mm long and trimmed to fit, with an anti-tamper security clip that resists handle manipulation. It is £14.00 with free UK delivery and is sourced from an overseas manufacturer.
- A second layer for when you are home: the Door Jammer – HOLDOOR hooks under the inside handle of a uPVC door and braces it against the floor, so the door stays shut even if the lock is snapped, bumped or picked. It is Sold Secure Bronze rated and Secured by Design accredited (Police Preferred Specification), adjusts from 70cm to 120cm for most UK single-handle uPVC doors (handle on the left or right), needs no drilling or tools and fits in about a minute. Made in Britain from British steel; invented and designed by Jon Gill in Manchester, with a wall mount; £90 with free UK delivery, 30-day returns and a 2-year warranty. It is not for sliding, French/double or patio doors.
Fire safety: In a fire: find the key, lift the Door Jammer off, unlock the door and leave the building.
FAQs
Can I replace a uPVC door lock mechanism myself?
Cylinders and handles, yes, for most people. Gearbox and full-strip replacement is possible DIY but needs careful measuring and patience; a locksmith is the safer choice if the door is your only exit or you are not confident.
How do I know which part has failed?
Use the door-open test. Stiff only when closed points to alignment; stiff when open points to the gearbox or strip; key turning with no effect points to the cylinder or gearbox.
Do I need to replace the whole door if the lock breaks?
Rarely. Most faults are fixed by a gearbox, cylinder, handle or spindle swap plus some adjustment.
What does PZ mean?
It is the distance between the centre of the handle spindle and the centre of the cylinder. Your handles and gearbox must match it.
Next steps
- Do the door-open test to find out whether you have an alignment problem or a failed part.
- Take the measurements above and photograph the faceplate before you order anything.
- Upgrade the cylinder to TS 007 3 star (or Sold Secure Diamond) while the door is apart.
- If your handle feels loose, consider the Security Spindle – HOLDOOR.
- For an extra layer when you are home, see the Door Jammer – HOLDOOR.
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