A euro cylinder — also called a barrel lock — is the standard lock mechanism fitted to most UK front doors, using a key-turned cam to operate the multi-point locking system on uPVC, composite, and aluminium doors. It's sold in three main types: double cylinder (key both sides), half cylinder (key one side only, common on sheds and garages), and thumbturn cylinder (key outside, turning knob inside for quick exit).

Main types

• Double cylinder — key required on both sides
• Half (single) cylinder — key on one side only, common on garages, sheds, and gates
• Thumbturn cylinder — key on the outside, turning knob on the inside for quick exit

Euro cylinders aren't a single standard size, so you need to measure your existing one before ordering a replacement. Measure from the centre of the fixing screw to the face of the door hardware on both sides — this gives you the internal and external measurements in millimetres (e.g. 35/45mm). For maximum security, the cylinder shouldn't protrude more than 5mm past the external face; if it does, size down.

How to measure a euro cylinder

1. Find the fixing screw — locate the central screw on the edge of the door, level with the cylinder
2. Measure the internal side — measure from the inside face to the centre of the screw hole
3. Measure the external side — measure from the centre of the screw hole to the outside face
4. Verify the measurements — check the total cylinder length equals internal + external

Cylinder sizes may be listed as internal/external or external/internal depending on the supplier — always check which measurement comes first before ordering.

Replacing a euro cylinder is a straightforward DIY job that takes about 10 minutes with just a screwdriver and your existing key. You don't need to remove the door handle to do it.

Steps to replace a euro cylinder

1. Remove the fixing screw — open the door and unscrew the central fixing screw on the door edge
2. Release the old cylinder — insert the key, turn slightly to align the cam, and slide the cylinder out
3. Align the new cam — insert the key into the new cylinder and turn until the cam sits flush
4. Fit the new cylinder — slide it into the door, making sure it is correctly oriented
5. Secure the screw — line up the fixing screw track and hand-tighten
6. Test before closing — check the lock works with the door open first, then test again with the door closed

Anti-snap protection is a security feature in high-security euro cylinders that uses a deliberate "sacrificial" weak point on the outer barrel. If an intruder tries to snap the lock, this section breaks away cleanly while the internal cam and locking mechanism stay intact and protected. Look for TS007 3-star or SS312 Diamond (Sold Secure Diamond) certification as confirmation a cylinder has been independently tested against this attack.

How anti-snap protection works

• Sacrificial cuts — grooves on the outer barrel encourage the metal to snap away from the main mechanism
• Intact cam — the central turning cam stays protected behind the break point, out of reach of tools
• Reinforced bars — high-end models add hardened steel bars or anti-drill pins that jam the lock if attacked