How To Fit A Mortise Deadlock


Always position a mortise lock in the centre of the door edge where it will give the strongest fastening. On some doors there is a specially reinforced point for fixing it. This will be marked on the door edge.

First wedge the door open so that is can be worked on from both sides. Some manufacturers provide a cardboard pattern for marking out the lock position on the door. Otherwise, use the lock body as a template to mark out the mortise. Place the pattern or casing against the side of the door and mark around its outer edge in pencil. Remember that the faceplate is recessed into the outer door How To Fit A Mortise Deadlockedge so if you are using the lock itself the mortise will in fact be approximately 4mm deeper than you have marked it.

Continue the lines onto the edge and other side of the door using a square or the lock casing body again.

Then set a marking gauge to half the width of the door and score a vertical line down the outer edge between the two marked out lines. If you don’t have a gauge, measure this distance with a rule.

The mortise should now be drilled and cut. When you have finished, try the lock body in position and make sure that the cover plate is level with the door edge.

Mark around the outside of the cover plate and remove the lock.

You will find it easier to cut the rebate if you first hold the lock in position and score around the outside of the plate with a trimming knife instead of marking with a pencil.

Chisel out a rebate so that the plate fits flush against the door edge.

Now mark and cut the keyholes. Hold the lock body against each side of the door in turn and push the point of the bradawl through the keyhole cut in the lock casing. This will mark the door with the exact position of each keyhole.

Remove the lock and with a 13mm drill bit bore two holes through to the centre from either side of the door. The keyhole shape can then be cut out by using a pad saw or — less neatly — with a 6mm chisel. Bear in mind that you can cut out a much bigger hole than is strictly needed since both keyholes are covered.

Once you have cut the keyholes, slide the lock case into position and try the key from both sides; adjust as necessary.

Screw the lock body to the door. With some locks you may have to remove the faceplate from the lock body first — it is usually held in place by two grub screws. Mark the screw positions and pre drill the holes with a bradawl. Screw the body into position (and replace the faceplate, if necessary). Then fit the keyhole covers (escutcheon plates) to the holes that you cut on either side of the door. There are normally two covers — make sure the one with the draught flap goes on the inside of the door. TIP: Mark the screw holes with the key in position. This ensures that the key fits neatly once the lock is in use.

Start the keyhole cover screws off by indenting the door with a bradawl. Screw each cover into position and remove the key.

All that remains is to fit the striking plate and bolt box on to the door frame. Careful marking out is required here, so that the lock operates freely and holds the door firmly once the lock is turned.

To mark the position of the rebate hold the door open and turn the lock. Remove the wedge, then push the protruding bolt against the frame and mark top and bottom with a pencil. This will tell you how deep to cut the rebate but not the exact position in which it has to be cut. Instead of relying on guesswork, it is possible to get a more accurate mark by shutting the door and turning the lock a number of times, as if you were locking the door. The result will be a slight indent where the end of the bolt strikes the inside of the door frame.

With a 12mm chisel chop out a squaresided hole to accommodate the bolt; the hole should be a few millimetres wider and deeper than marked to allow both for the slightly larger bolt box and the fact that the striking plate must be rebated into the door frame.

Once the bolt hole is cut, slide the plate into position and mark around the outside with a pencil. As with the lock body you will find it easier to cut a rebate if you run a trimming knife around the outside of the striking plate first. Chisel out the rebate then mark and predrill the screw holes with the bradawl. Before securing the plate, make sure that the door will close properly.


All mortises — particularly those cut in a relatively narrow piece of wood such as the edge of a door — need to be as accurate its possible.

First select a drill bit the same diameter as the width of the lock (normally 13mm). Bore a series of closely spaced holes along the centre line on the door edge. The holes should be the same depth as the lock body (including the faceplate). Rather than relying on guesswork, you can gauge whether you have reached the required depth by wrapping a piece of tape around the bit; the distance between the

tape and the point of the drill should be the same as the depth of the lock body.

After drilling, pare away the remaining wood with the chisel. Aim to make the sides of the hole as square as possible.

Stop occasionally to test whether the lock fits.

Bore a series of closely spaced holes, preferably with a hand brace, Drill slightly above and below the line to give adequate clearance for the lock Pare down both sides of the mortise with the chisel. Try not to remove too much at once otherwise you could severely weaken the door.

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