Seven tips for fitting mortise locks

1. Measure to fit: look at the width of the door stile and the thickness of the door before buying a mortise lock. Lock bodies come in several different thicknesses and lengths. This is important if you are buying a lock for a glazed door, which may have very narrow stiles.

2. Mark the keyhole: when you have cut the mortise and the faceplate recess, hold the lock body against the face of the door so you can mark the position of the keyhole on it with a bradawl or pencil.

3. When to stop drilling: drilling too deeply into the door stile can weaken the door and, if it’s glazed, you could also crack the glass. To avoid this, wrap some tape round your drill bit so you can see when you’ve drilled holes to the right depth — a fraction more than the length of the lock body. If your drill has a depth-stop attachment, you can use that instead.

4. Don’t split your sides: choose a drill that’s just a fraction wider than the thickness of the lock body. If you create a wider mortise, you will weaken the door so much that it could split around the lock if forced.

5. Getting a firm grip: turn the key to extend the bolt of the lock before you insert it into the mortise to test the fit. If it jams you can then grip the bolt with pliers and pull the lock body out again.

6. Measure up for a perfect fit: with the lock fitted, measure the distance between the bolt and the closing face of the door. Use this measurement to position the keep plate on the door frame so that the bolt just engages in the recess when the door is locked.

7. Avoid an eyeful of dust: resist the temptation to blow into the mortise to clear out loose shavings — you can easily end up with an eyeful of splinters and wood dust. Pull out the waste with the drill bit or scrape it out with a narrow chisel.

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