How do I change a door lock barrel?

Measuring a euro cylinder correctly, removing the old one, and choosing a replacement that is actually secure.

Difficulty: intermediate
Time: 45 min
Read: 3 min
Safety: caution

Short answer

Measure from the fixing screw to each end separately — euro cylinders are often offset, and quoting a single total length gets you the wrong part. Remove the screw in the door edge, turn the key slightly, and the barrel slides out.

Changing a euro cylinder is one of the more useful DIY jobs: it is what you do when you move house, lose a key, or want to upgrade to an anti-snap lock. It takes ten minutes once you have the right part.

Safety

Do not remove the lock while the door is closed — you may not be able to open it again. Work with the door open, and have the replacement barrel to hand before removing the old one. If the door is your only entrance, do this during the day when a locksmith is available.

What you'll need

Replacement cylinder
Screwdriver
Tape measure (optional)

Step by step

Measure the existing cylinder correctly.From the centre of the fixing screw hole to each end separately, for example 35mm and 45mm. Cylinders are often offset and a single total is not enough.
Note whether it is single or double, and thumbturn or key.Double cylinders take a key both sides; thumbturn cylinders have a handle inside, which is safer in a fire but less secure if there is glass nearby.
Choose an anti-snap cylinder to a recognised standard.Older cylinders can be snapped in seconds. An anti-snap, anti-bump, anti-drill cylinder to a recognised kitemark or standard is a genuine security upgrade for very little money.
Open the door and keep it open throughout.With the cylinder out you cannot operate the lock. Never do this with the door closed.
Remove the long screw in the edge of the door.It runs through the lock case into the cylinder. It is usually the only fixing holding the barrel in.
Turn the key a few degrees and slide the cylinder out.The cam inside must line up with the lock body. A slight turn of the key — usually about 10-15 degrees — releases it.
Fit the new cylinder the same way and refit the screw.Insert with the key turned slightly, align the fixing hole, and refit the screw. Do not overtighten.
Test thoroughly with the door open before closing it.Lock and unlock several times from both sides, and check the handle operates the multipoint mechanism properly.

Tips

A cylinder should sit flush or barely proud of the handle escutcheon. One protruding more than 3mm can be gripped and snapped.
Buy cylinders keyed alike if you are changing several doors, so one key operates all of them.
Keep the old cylinder. If the new one is wrong, refitting the original gets you secure again while you sort it out.

Common mistakes

Measuring the total length only — Euro cylinders are usually offset. A 70mm cylinder could be 30/40 or 35/35, and the wrong split leaves it protruding or recessed.
Removing the cylinder with the door closed — You cannot operate the lock without it, and you may be unable to open the door at all.
Fitting a cylinder that protrudes from the handle — A protruding cylinder can be gripped and snapped, which is a well-known and quick method of entry.

Questions people ask

How do I measure a euro cylinder?

Measure from the centre of the fixing screw hole to each end separately, since cylinders are usually offset. A total length alone is not enough to order the right part.

Are anti-snap locks worth it?

Yes. Older euro cylinders can be snapped in seconds, and it is a common entry method. An anti-snap cylinder to a recognised standard costs little and is a real upgrade.