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How to fit a lock

Difficulty rating: Medium

These tasks may be tricky so will suit you if you're experienced in DIY, or simply love a challenge.

Before you get started on any of our 'how to' guides, please take a moment to read through our DIY safety tips.

1. Choosing a lock: look for the kite mark

The best and strongest locks have a kite mark and are passed to British Standard BS3621. Most insurance companies insist you have these. They're also recommended by crime prevention officers.

2. Best locks for external doors

  • The more levers, pins or discs a lock has, the more difficult it is to pick.
  • The main locks on your exterior doors should be to BS3621 standard. Most insurance companies demand this level of security.

Front door

  • When possible fit two locks on your front door - both to BS3621 standard. Position a mortice deadlock (fig. 2) about halfway up the door (fig. 1B).
  • And fit a night latch on your front door (fig. 3) one-third of the way down (fig. 1A). To install this lock, see Section 4 below.
  • If you only fit one lock, go for an automatically dead-locking night latch made to BS3621 standard (fig. 18).

Back door

  • Protect your back door with a mortice sash lock to BS3621 (fig. 4)

Additional security - bolts

  • It's a good idea to fit hinge bolts, especially to doors that open outwards as the hinge knuckle pins are a weak point (fig. 5). Position them 150mm below the top hinge and 150mm above the bottom hinge (fig. 1D).
  • Strengthen your back door with security mortice bolts (fig. 6).
  • Follow the manufacturer's instructions to install these bolts.
Helpful hint...

When drilling for the mortice recess, use a piece of tape on your drill bit to mark how deep you should drill each time.

Where to position your locks
Figure 1
A mortice deadlock.
Figure 2
Night latch.
Figure 3
Mortice sash lock.
Figure 4
Hinge bolts.
Figure 5
Security mortice bolt.
Figure 6

3: Fitting a mortice deadlock and a mortice sash lock

  • Don't fit the lock at the rail joint as this will weaken the door (fig. 1). Position it about halfway up the door where the timber is solid (fig. 1B).
  • Hold the door in the open position and use either the manufacturer's template (if supplied) or the body of the lock to mark the mortice's position on the edge of the door (fig. 7).
  • Set a marking gauge to half the thickness of the door (fig. 8) and score a line down the door edge to mark the position of the lock.
  • Drill holes on this centre line the same width as your lock (fig. 9).
  • Chisel away a small amount of wood at a time to open the drill holes into a clean rectangular hole to fit your lock (fig. 10).
  • When the mortice lock space is chiselled out, insert the lock and outline the faceplate with a marking knife (fig. 11).
  • Chisel a recess for the faceplate (fig. 12). Then mark and drill screw holes to fix it in place.
  • Use the lock body to mark the position for the keyhole (fig. 13).
  • Use a drill and a thin-bladed pad saw to form the keyhole (fig. 14). Check the keyhole for fit from both sides of the door. Install the lock body and screw on the faceplate.
  • With the bolt out and held against the door frame, mark the position of the striking plate and the depth of the bolt box (fig. 15). Use these marks to position the striking plate on the inside surface of the door frame.
  • Recess the plate into the door frame. Cut out the wood to receive the bolt. Drill holes for the recess plate screws (fig. 16).
  • Before you fix the plate make sure the door closes properly.
  • Fit the key plate over the hole and place the key in the lock with the key plate loose on the shaft of the key. Fix the key plate in place.
  • The green/yellow earth wire connects both to E and to the terminal of the mounting box (fig. 3).
  • Tighten terminal screws securely and give wires a good tug to check they're firmly fastened.
Use the body of the lock as a template to mark the position of the mortice
Figure 7
Mark the position of the drill holes for the lock by scoring a line with a marking gauge down the centre of the door edge.
Figure 8
Use an electric drill to bore holes to the width of the lock along the marked center line.
Figure 9
Form the recess for the lock with a chisel. For the best result only cut away a small amount of wood at a time.
Figure 10
Chisel the mortice clean, insert the lock and mark the outline of the faceplate with a marking knife.
Figure 11
Chisel a recess for the plate.
Figure 12
You can use the lock body as a guide to mark the position of the keyhole.
Figure 13
Use a drill and thin bladed pad saw to cut the keyhole.
Figure 14
With the bolt out and held against the door frame, mark the position of the striking plate and the depth of the bolt box.
Figure 15
Mark and drill holes for the recess plate screws.
Figure 16

4: Fitting a night latch

  • Read the manufacturer's instructions before starting this job.
  • Mark and drill the hole for the cylinder through the door stile (fig. 1A) about one-third of the way down from the top of the door.
  • The diameter and position for the hole will be in the manufacturer's instructions, usually shown as a template.
  • Place the slip ring B over the cylinder C and insert it into the hole in the door. Then place the fixing plate D over the hole. Secure it with connecting screws A, so the cylinder and fixing plate are upright (fig. 18).
  • If you haven't already done it, take the key out of the cylinder.
  • Check the arrows on the thimble E and lock cover plate F line up (fig. 17).
  • Place the case I over the fixing plate, making sure connecting bar G enters the slot in thimble E. The connecting bar often needs shortening – you can do this with a hacksaw (fig. 18).
  • You might need to mark and cut the edge of the door if the lock cover plate F has a lip. Use a chisel for this (fig. 18).
  • Replace the case and fix screws H to attach the case to the door.
  • Line up the striking plate (the metal piece on your door frame that the lock bolt enters) with the lock bolt J. Mark its position on the door frame and chisel out the wood for a flush fit. Fix it in place with the screws provided (fig. 18).
Helpful hint...

Make sure all your door frames are in good condition or they'll make your locks ineffective. If the frame can be moved or rocked when you push it, either add extra wall fixings, or replace or repair the frame.

Check the arrows on the thimble and lock cover plate line up.
Figure 17
A typical night latch
Figure 18

5: Replacing a Euro cylinder lock

Some locks are so good they'll stop intruders in their tracks. For example, PVCU composite doors are fitted with Euro cylinder locks. Put one on your door and you'll immediately upgrade the security of your home.

Look for the latest security standard - BSEN1303:2005 - that tests against all methods of beating a lock. The cylinder also has Secured by Design accreditation - a police-preferred specification that focuses on crime prevention.

6: Replacing a rim cylinder lock

Replacing a rim cylinder on an external timber door is an easy and effective way to secure your home without having to change the entire lock or door.

Upgrade to a British standard rim cylinder and you can be confident the lock been tested to withstand all known ways to break it.

7: Replacing a handle

If your door handle is stiff or it's tarnished and dated, you may need a replacement.

Yale's PVCU replacement handle is easy to fit, and has a multi-point lock system for maximum security. To start off you'll need to find out the size of your lock.

Tools for the job


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