How do I fit a mortice lock into a timber door?

This is the most demanding piece of ordinary door carpentry there is, and it is entirely doable with hand tools if the marking out is right. Everything that goes wrong goes wrong at the marking stage or at the drilling stage, so both are worth taking slowly. If the door is a hollow-core or a modern engineered door, stop and check what is inside it first.

Safety

What you'll need

• The lock, with its keep and faceplate
• A marking gauge and a try square
• An auger or flat bit matching the lock case thickness
• A drill with a depth stop or tape marker
• Sharp bevel-edge chisels, 12mm and 25mm
• A brace or a slow-speed drill for the through-holes (optional)
• A sharp marking knife and a pencil

Step by step

1. Check the door will take the lock.A solid timber or solid-core door will. An engineered or hollow-core door has a solid lock block, usually a few hundred millimetres tall, on one edge only — find it by tapping, and if the intended lock position is outside it, the lock cannot go there.
2. Set the height.Conventionally the lock centre goes level with the middle rail on a panelled door, or at a comfortable height around waist level. On a front door with a nightlatch above, the deadlock goes lower, and it must not clash with a rail joint or a panel groove.
3. Mark the case outline on the door edge.Hold the lock against the edge in position and mark the top and bottom of the case with a knife. Then set a marking gauge to half the door thickness minus half the case thickness and gauge from both faces — two lines that should meet the case width exactly. Working from both faces is what keeps the mortice central.
4. Mark the depth.The mortice must be a few millimetres deeper than the case so the lock never bottoms out. Set a depth stop on the drill or wrap tape round the bit at that depth, and check it against the lock.
5. Drill out the waste.A row of overlapping holes down the centre of the gauged lines, the bit's diameter matching the case thickness as closely as possible. Drill square to the edge — clamp a block alongside as a guide if you are not confident, because a mortice that wanders sideways will break out through a face.
6. Chop the mortice square.Pare the sides back to the gauge lines with a chisel, bevel towards the waste, and square the ends. Work from both ends towards the middle. Test-fit the lock frequently rather than removing too much.
7. Drill the spindle and key holes from both faces.Slide the lock into the mortice, mark the centres of the follower and the keyhole on both faces of the door using the lock itself. Drill from one face until the point just breaks through, then finish from the other side. Drilling straight through in one pass tears out the far face.
8. Shape the keyhole.Drill the round part, then pare the slot below it with a narrow chisel from both faces. Keep it small — the escutcheon covers a neat hole and exposes a ragged one.
9. Fit dry and mark the forend recess.Push the lock fully home, hold it square, and knife round the faceplate. Remove the lock and pare a shallow recess so the faceplate finishes flush with the door edge, no more.
10. Fix the lock and test before the keep.Two screws through the forend. Fit the spindle, handles and key, and work everything with the door open. The latch and the bolt should move freely with no binding.
11. Mark and cut the keep on the frame.Close the door, throw the bolt gently against the frame to mark it, or smear the bolt end with lipstick or chalk. Mark the latch position the same way. Cut mortices in the frame for both, then recess the keep plate flush.
12. Fit the escutcheons and check the whole thing.Key turns freely both ways, latch springs cleanly, door closes without a shove, bolt enters the keep without touching the sides. Any binding now becomes a jam in winter.

Common mistakes

• Gauging the mortice from one face only — Any inaccuracy in the gauge setting puts the whole slot off centre, and an off-centre mortice leaves a thin wall of timber on one side that splits under the first hard closing.
• Drilling the through-holes in one pass — The bit blows out the far face in a ring of splinters that no escutcheon will cover. Drill from both sides to a meeting point.
• Cutting the forend recess before test-fitting — If the mortice is a fraction shallow the lock will not go fully home, and a recess cut in the wrong place cannot be moved.
• Fitting a mortice lock to a hollow-core door — There is nothing to cut into outside the lock block, and the case will simply be sitting in a cardboard honeycomb.

Questions people ask