Cylinder Locks Explained: How to Measure, Choose & Replace Yours

Cylinder Locks Explained: How to Measure, Choose & Replace Yours

Whether you're a trade professional fitting a replacement on a composite door or a homeowner who's just lost a key, understanding cylinder locks is essential. Getting the right type, the right size, and the right security rating means the difference between a door that's genuinely secure and one that's vulnerable. Often without the occupant knowing. This guide covers everything you need to know about cylinder locks: how they work, how to measure them correctly, which security ratings to look for, and when to upgrade.

What Is a Cylinder Lock?

A cylinder lock. Also called a barrel lock or euro cylinder is the core locking component fitted into doors that use a multipoint lock system. It's the part you insert your key into. The cylinder houses a pin tumbler mechanism: When the correct key is inserted, pins of varying heights align inside the shear line, allowing the barrel to rotate and operate the locking mechanism.

Cylinder locks are used in uPVC doors, composite doors, aluminium doors, and many timber doors fitted with a multipoint or mortice lock. They are not typically found in traditional Yale-style rim nightlatches, which use a separate rim cylinder.

Types of Cylinder Lock

Understanding which type you need before purchasing saves time and repeat trips to the merchant.

Euro Double Cylinder

The most common type for external doors. Has a keyhole on both sides, so the door can be locked and unlocked from the outside or inside using a key. Suitable for doors where you need to lock up when you leave.

Thumb Turn Cylinder

Has a keyhole on the outside and a thumb turn knob on the inside. Easier and faster to open from inside - particularly useful for fire escape routes or properties where quick internal exit is required. Not recommended where a window is within arm's reach of the inside turn, as it can be a security risk.

Single Cylinder

Key-operated on one side only. Less common for external doors but can be found in some back door and internal applications.

Trade Tip: When specifying cylinders on new-build or social housing contracts, always check whether the project requires master-keyed or keyed-alike suites. Avocet and Yale both offer keyed-alike packs within their security-rated ranges - ideal for landlords or developers managing multiple properties.

How to Measure a Cylinder Lock

Fitting the wrong-size cylinder is one of the most common mistakes on door hardware jobs. An oversized cylinder protrudes beyond the door faceplate, leaving the exposed cam vulnerable to snapping - one of the most prevalent door attack methods in the UK.

Cylinder locks are measured in millimetres from the centre of the screw hole to each end. This is written as an external measurement / screw centre / internal measurement. For example, a 35/10/35 cylinder measures 35mm from the outside face to the centre of the fixing screw, and 35mm from the screw centre to the inside face.

Common sizes and what they're used for:

30/10/30 - suited to slim or flush-profile doors, and some older aluminium systems.

35/10/35 - the most widely fitted size, covering the majority of standard uPVC and composite doors.

40/10/40 - used on thicker door profiles, including some heavier composite constructions.

30/10/40 or 35/10/45 - offset sizes for doors where the cylinder sits closer to one face of the door than the other.

To measure accurately: Remove the existing cylinder by unscrewing the fixing screw through the edge of the door (usually an 8mm cross-head screw accessed from the door side), then withdraw the cylinder with the key. Measure from the centre of the screw hole to each end.

Cylinder Lock Security Ratings: What TS007 and SBD Actually Mean

Not all cylinder locks offer the same level of protection. The key rating standard in the UK is TS007:2014, which grades cylinders from 1 star to 3 stars. The higher the rating, the greater the resistance to the most common forms of attack: snapping, drilling, bumping, and picking.

TS007 Star Ratings at a Glance

1 Star (TS007 1*): Meets baseline security requirements. Anti-snap, anti-bump, anti-pick, and anti-drill protection. Suitable for most residential applications when combined with a 1* handle and 1* letterplate (known as the "3+1+1 rule”).

3 Star (TS007 3*): The highest single-component rating. Provides full TS007:2014 compliance as a standalone cylinder - no need to combine with additional star-rated hardware. Products like the Yale Platinum 3* and Avocet ABS Master 3* fall into this category.

Secured by Design (SBD) is the UK Police's crime prevention initiative. SBD-accredited cylinders have been independently tested and meet the physical security requirements demanded by planning conditions on new residential developments and social housing specifications. If you're working on a PAS 24-compliant door set, the cylinder must form part of an approved combination.

Why Anti-Snap Matters and What to Look For

Lock snapping remains one of the most common forced entry methods targeting euro cylinder doors in the UK. An attacker uses a tool to snap the exposed part of an oversized or non-snap-resistant cylinder, gaining direct access to the internal cam mechanism. Often in under 30 seconds.

Anti-snap cylinders include a designated sacrifice point. A sacrificial break line. Positioned just beyond the door's faceplate. If the outer portion is snapped off, the inner lock mechanism remains intact and fully operational. The door remains locked and inaccessible.

Look for cylinders that are rated anti-snap as a minimum. Any TS007 1* or 3* rated cylinder must, by definition, include anti-snap protection. Cylinders without a TS007 rating offer no guarantee of anti-snap performance.

Trade Tip: Always advise customers to check for cylinder protrusion before leaving a job. If the cylinder extends more than 3mm beyond the door's outer faceplate, it remains vulnerable to snapping regardless of the cylinder's own rating. Size it correctly - or fit a sacrificial external plate - and document it.

Replacing a Cylinder Lock: Step-by-Step

1. Replacing a euro cylinder is one of the most straightforward lock jobs. No specialist tools required for a standard single-door installation.
2. Open the door and locate the fixing screw in the edge of the door. This is typically an 8mm cross-head or hex screw, set midway into the door thickness.
   
3. Undo the fixing screw two or three turns. You don't need to fully remove it.
   
4. Insert the key and turn it slightly to retract the cam (the internal tongue). This disengages the cylinder from the multipoint lock body.
   
5. Slide the cylinder out from the front or back of the door.
   
6. Measure the old cylinder and purchase a direct replacement of the same dimensions and type.
   
7. Fit the new cylinder in reverse order. Ensure the cam engages correctly with the lock body before tightening the fixing screw.
   
8. Test operation with the door open before relying on it closed.

Need to replace your cylinder? FixTrade.co.uk stocks a full range of euro cylinder locks from Yale, Avocet, and ERA - including TS007-rated anti-snap cylinders, thumb turn cylinders, and keyed-alike packs. View the full cylinder lock range.