Door Lock Guides and How‑Tos
Accurate sizing prevents poor fit, weak security, and unnecessary returns. These step‑by‑step guides explain measurements, standards, and practical tips so you can order with confidence or brief your installer clearly.
Before You Start
Have a steel ruler or calipers, pencil, and masking tape Keep the door open and supported while working Wear eye protection when drilling or chiselling
How to Measure a Euro Cylinder
Euro cylinders are measured in two halves from the central fixing screw. Record the internal length first, then the external length. The correct cylinder should sit flush with the escutcheon without protruding more than a couple of millimetres on the outside. Protruding cylinders are easier to attack and may invalidate insurance. If your door uses a thumbturn inside, order a cylinder that matches that function. For communal entrances, consider anti‑snap TS007 3 Star models for added protection.
- Remove the fixing screw at the edge of the door and gently slide the cylinder out while turning the key
- Measure from the centre of the screw hole to each end to get internal and external lengths, for example 35/45
- Check cam type and finish to match existing hardware
- Reinstall the cylinder and ensure smooth locking before closing the door
Checklist
Internal length External length Keyed alike required? Thumbturn or key/key
Mortice Lock Sizing Guide
Mortice locks for timber doors have two key dimensions: backset and case depth. The backset is the distance from the forend edge to the spindle centre, commonly 44 or 57 mm in the UK. Case depth is the overall body size set into the door. Choose BS3621 certified locks for front doors to meet most insurance policies. Before replacing, confirm handedness and the position of keyhole and follower to ensure your handles and key escutcheons align after fitting.
- Remove the existing faceplate screws and slide the lock case out
- Measure backset from the forend edge to spindle centre
- Measure case depth and forend height/width
- Check the keep and door frame for wear or misalignment
Common Backsets
Smart Lock Compatibility
Most retrofit smart locks fit doors using euro cylinders with a protruding thumbturn space. Check clearance around the handle rose and any nearby mullions. Battery housings may require extra depth on the inside. Verify app ecosystem support and encryption standards, and plan how you will share temporary access with guests. For rented properties, confirm permission before modifying hardware and keep an emergency mechanical key available.
Ensure the cylinder can be operated from both sides when a key is inserted Plan battery replacement and emergency power options Check app compatibility with your phone and hubs
Fit Check
FAQs
What is TS007 3 Star?
Do I need a locksmith to install a new cylinder?
What is BS3621?
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