How do I fit a new mortice lock into an old mortice?
Measuring the lock you already have so the replacement drops straight in, and the three ways an almost-right lock leaves visible damage on a door.
- Difficulty
- intermediate
- Time
- 1 hr
- Read
- 4 min
Short answer
Take the old lock out and measure four things: case size, backset, case thickness and forend shape. Match all four and the new lock goes straight in. If the keyhole or spindle position differs by even a few millimetres, you are drilling new holes in the door face, so match rather than compromise.
Replacing a mortice lock is a far easier job than cutting a new mortice, provided the replacement is genuinely the same size. The traps are all dimensional, and they are all discoverable in ten minutes with the old lock on the bench in front of you.
What you'll need
- A steel rule and a tape
- Screwdrivers
- A sharp chisel for adjusting the recess (optional)
- Light machine oil
Step by step
- Remove the old lock intact.Handles and spindle off, escutcheons off, then the two forend screws. Pull the lock out by the faceplate — a screw threaded into one of the forend holes gives you something to pull on if it is tight. Do not lever against the door edge.
- Measure the case size.The overall length of the steel body, not including the forend's projection. This is the figure locks are sold by, still usually in inches — two and a half or three inches are the common domestic sizes.
- Measure the backset.From the front face of the forend to the centre of the keyhole. On sashlocks this also sets the handle position. Common backsets are around 44mm and 57mm, and this dimension is the one that cannot be fudged.
- Measure the case thickness.Across the steel body. It must be no thicker than the existing mortice or the slot has to be widened, which means chopping a slot that is already there and easy to overcut.
- Note the forend shape and size.Square or radiused, and its length and width. A radiused forend in a square recess leaves two visible gaps at the ends; a square one in a radiused recess will not sit flat. Some locks are supplied with both faceplates in the box.
- Check the keyhole and spindle positions.Hold old and new together with the forends aligned. The follower and keyhole must be at the same height and the same backset. Any difference means new holes through the door face, which will show around the escutcheon.
- Check the lever count and the standard.If this is an external door, take the opportunity to fit a lock that meets the relevant British Standard, with the kitemark on the faceplate. It is the same amount of work and it is what an insurer asks for.
- Fit dry and check before screwing.Slide the new lock in and see whether the forend sits flush without forcing. If it stands proud, pare the recess a shaving at a time. If it sits below the surface, do not pack it — the screws will pull it in and the door edge will dent.
- Fit the spindle and key and test.Work the latch and bolt a dozen times with the door open. Then close the door and check the bolt enters the existing keep cleanly. A new lock frequently projects a fraction differently, and the keep may need easing or refixing.
- Fit new escutcheons if the old ones do not cover.Slightly different keyhole shapes are common between makes. A larger escutcheon covers a multitude, and a pair costs very little.
- Oil the case and record what you fitted.A drop of light machine oil through the forend slots. Write the make, case size and backset down — the other doors in the house probably match.
If it doesn't work
The new lock will not go fully into the mortice
Cause: The mortice is shallower or narrower than the new case — Fix: Measure to find out which. Depth is easily deepened with a long chisel or a drill; width has to be pared very carefully because the mortice must stay central.
The key is stiff after fitting
Cause: The lock is being distorted by overtight forend screws, or the keyhole in the door is fouling the key — Fix: Slacken the screws and try again. If that cures it, ease the keyhole slot slightly rather than overtightening to hold the lock.
The door no longer latches after the swap
Cause: A slightly different latch projection or a keep that was already marginal — Fix: Adjust the keep rather than the lock — that has its own established fix, and it is the right place to take up the difference.
Questions people ask
Can I fit a bigger lock into a smaller mortice?
Only by enlarging the mortice, which is real work and risks going off centre. Where possible, match the case size exactly.
The old lock is unbranded and ancient. What do I do?
Measure it and buy on dimensions — case size, backset, thickness. Modern locks are made in the traditional sizes precisely because so many old doors need them.
Should I upgrade to a euro-profile mortice lock?
It is worth considering if you want the timber door keyed alike with uPVC doors, but the case dimensions are different from a lever lock, so expect to adjust the mortice and drill a new cylinder hole.