How do I replace the multipoint lock in a uPVC door?

Getting the old lock strip out, fitting the new one without cross-threading the faceplate, and the alignment check that stops the new lock failing the same way as the old one.

Difficulty: intermediate
Time: 1 hr 30 min
Read: 4 min
Safety: caution

Short answer

With the door open: remove the handles and the cylinder, take out the faceplate screws top and bottom, then draw the whole strip out of the door edge. Fit the new one in reverse, screwing the faceplate loosely first, and check every bolt lines up with its keep before tightening anything or testing with the door shut.

The job itself is straightforward — the lock slides out of a routed slot in the door edge — but the two ends of it are where people come unstuck: ordering the right part, and dealing with the alignment problem that broke the old lock in the first place. If you fit a new gearbox into a door that is still dropped, you will shear the new one too.

Safety

Do not start this job on a door that has to be secure that night unless the replacement part is already in your hands and you have checked it against the old one. If the door is currently locked shut and the gearbox has failed, stop — that is a locksmith's job, and forcing it usually damages the door as well.

What you'll need

The correct replacement lock, checked against the old one
Pozidriv screwdrivers
A hex key set for the hinges
A new euro cylinder if the old one is worn (optional)
Light machine oil or a PTFE lock lubricant

Step by step

Work with the door open and supported.Wedge it so it cannot swing. Everything in this job is done from the closing edge, and a door that moves while you have the lock half out will damage the faceplate routing.
Remove the handles.Take out the two through-bolts, lift both handle plates away and slide the square spindle out. Keep the bolts and the spindle together — you will reuse them unless the handle is being changed as well.
Remove the cylinder.One long screw enters the door edge through the faceplate, below the cylinder. Take it out, put the key in the cylinder and turn it a few degrees each way until the cam lines up inside the profile, then the cylinder slides out towards you.
Take out the faceplate screws.There is a screw at the top and bottom of the faceplate and often one or two more behind bolts. Remove them all and keep them — replacement locks are frequently supplied without screws, and the originals are the right length for the door.
Draw the lock out.Grip the faceplate at the gearbox end and pull it squarely out of the door edge. It should come with steady pressure. If it binds, check that no bolt is still thrown and that no screw remains. Never lever against the uPVC — it marks permanently.
Compare old and new side by side before fitting.Lay them together on the floor. Overall length, faceplate width, backset, PZ, bolt positions, bolt types, end shape. If anything is different, stop and sort the part out now, not with the door in pieces.
Slide the new lock in and start the screws by hand.Feed it straight into the slot until the faceplate sits flat. Start the top and bottom screws by hand — the fixing holes in a uPVC door edge are into a steel reinforcement or into plastic, and a power driver will strip either. Leave them loose.
Refit the cylinder and the handles.Cylinder first, with the cam correctly aligned, then its fixing screw finger-tight plus a little. Then the spindle and both handles, bolts finger-tight. Nothing is fully tightened yet.
Test everything with the door still open.Lift the handle. Every bolt should throw smoothly with no crunching. Turn the key. Drop the handle. Repeat a dozen times. If it is anything other than smooth now, it will never be right with the door shut.
Check bolt-to-keep alignment before closing the door.Throw the bolts with the door open, then bring the door up to the frame slowly and look at whether each bolt lines up with the mouth of its keep. Correct any misalignment at the hinges before you ever try to lock it closed.
Tighten in the right order.Faceplate screws snug, not hard. Then handle bolts, evenly, until the plates just stop moving — overtightening a handle bolt binds the spindle and makes a new lock feel stiff. Then the cylinder screw.
Lubricate and record what you fitted.A little light machine oil or a PTFE spray on the bolts and into the gearbox slot. Then write the make and reference of the lock, the cylinder size and the handle centres somewhere you will find them — inside a kitchen cupboard door is traditional and it works.

If it doesn't work

The new lock will not slide fully in

Cause: The routing in the door edge is slightly shorter than the new faceplate, or a bolt is catching the slot — Fix: Check every bolt is retracted. If it is genuinely the routing, the slot can be lengthened carefully with a sharp chisel or a router — but confirm the part is right first, because this is usually a wrong-part symptom.

The handle is stiff after fitting a new lock

Cause: Handle bolts overtightened, or the spindle binding in the follower — Fix: Slacken both handle bolts fully, work the handle, then retighten evenly just until the plates stop moving. If it is still stiff, the spindle may be a fraction long and holding the plates apart.

It works open but jams when the door is shut

Cause: Alignment — the door has dropped or the frame has moved — Fix: This is what killed the old lock. Adjust the hinges to bring the bolts back level with the keeps, then set compression at the keeps. Do not use the lock to pull a misaligned door into place.

Checklist

Replacement part compared side by side with the old lock
Original faceplate screws kept
Cylinder cam aligned before refitting
Handle tested a dozen times with the door open
Every bolt lines up with its keep before the door is closed
Hinges adjusted if alignment was the original cause
Lock make, reference and cylinder size written down