How to install a mortise lock?

It is quite straightforward to install a mortice lock to a wooden door but it does require accurate measurements and cutting so that the mortice lock and striker plate install correctly.

Two sorts of mortice lock are available:

• Sashlock - which has a door catch and lock bolt - so has a keyhole and door handle spindle.

• Deadlock - which just has a lock bolt - without the door catch or door handle spindle - so just a the keyhole.

1. Decide on the height on the door for the mortice lock; if in doubt, look at other locks in the property - ideally all the door handles should be the same height from the floor. On doors with a horizontal mid rail, the lock should be positioned clear of the mid-rail joints otherwise the joints will be weakened, the lock should be installed in the centre of the rail to clear the upper and lower joints.

Place the body of the mortice lock against the side of the door and mark the top and bottom edges of the main body on the side of the door.

Also mark the positions of the handle spindle and the keyhole by marking through the body of the mortice lock - allow for the thickness of the front plate of the lock as this be recess into the edge of the door.

2.Drill holes in the door for the handle spindle and keyhole - the hole for the handle spindle hole will need to be at least as big as the spindle boss of the lock - for the keyhole, drill the top hole and then cut out the rest of the shape using a thin pad saw. 

Using a carpenters square on the door edge, mark the top and bottom positions of the mortice lock body to the edge of the door.

Mark the vertical centre line of the door thickness and from this mark, mark out the width of the mortice lock body equally either side of the centre line. Use a sharp chisel to mark along the lines for the body of the lock to establish the edges - by cutting the surface of the wood, this should prevent any wood lifting from the surface beyond the lines required.

Drill a series of holes into the edge of the door using the marks as a guide on the edge of the door - this will create a rough hole to take the body of the mortice lock (by using a drill, most of the unwanted material will be removed quite easily and quickly). Ensure that each hole is at least as deep as the body and front plate of the lock. Take care to keep the drill parallel with the sides of the door - if the hole is not square to the edge, the mortice lock may not fit square in the door, and the drill bit could break through the side of the door. 

3.After all the holes have been drilled in the edge of the door, tidy up the hole by removing waste wood and roughly smooth the sides of the hole using a sharp chisel.

Once the hole has been tidied up, check whether the body of the lock fits - if necessary, identify and remove any area of wood restricting the fit - the fit needs to be a slide fit without too much slack. Ensure that the body of the lock fits into the door square to the edge - so that the front plate of the lock fits squarely onto the edge of the door, and that the sides of the front plate are parallel with the sides of the door.

4.Once the hole for the body of the lock has been finished, push the lock into the door with the front plate against the edge - mark around the front plate onto the edge of the door.

Remove the lock from the door and use a sharp chisel to mark along the lines for the front plate of the lock to establish the edges - by cutting the surface of the wood, this should prevent any wood lifting from the surface beyond the lines required.

Cut out a rectangular recess in the edge of the door to take the front plate of the lock using a sharp chisel.

Fit the lock back in the door so that the front plate is flush with the edge - check the positions of the holes for the key hole and door handle spindle from both sides of the door - if necessary, remove the lock and adjust the size and/or position of the holes so that they line up with the appropriate parts of the mortice lock body.

When everything lines up, refit the mortice lock into the door and secure it using screws through the front plate.