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My house has cylindrical locks on all the internal doors. These are the ones with one keyed side and one knob/toggle side to lock and unlock a door without the key.

I live in South Africa, and for security purposes I would like to lock the internal doors for the rooms I am not using. (This is legal here). Here mortise locks are ubiquitous and cheap.

I have been researching my options and it seems they are:

  • Leave the existing knob and install a cylindrical deadbolt. (More expensive, but easier)
  • Remove the existing lock and replace with a mortise lockset. (Cheaper but more difficult)

I want to go with the mortise locks because they are cheap, easy to come by, and I like the look of a handle rather than a knob, and the existing doorknobs need to be replaced (some of them are simply old and broken). However, I'm not experienced at woodwork/DIY and don't know how easy this is to do. I could drill a hole, but am not so sure about being able to get the square mortise lock cut-out correct. Additionally, I am concerned about the existing cylindrical cutout being too large for the mortise lock handle-plate.

Is there a clear winner here, and am I overlooking anything? I would be willing to go with the cylindrical deadbolt option if it is significantly simpler to install.

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Fitting a mortise-lock is fairly straightforward and only requires basic woodworking skills, really. The two most important tools you'd need are a drill bit of right size for the lock and a sharp chisel of the same width (an additional wide chisel for the sides is useful, but not essential).

Regarding the faceplate width, you'll have to check the size of your existing holes against the size of the plates on the handles you want to use, allowing for the difference in centre position between the cylinder lock and the handle of the new mortise-lock.

Assuming that you can find handles/faceplates to fulfill this requirement, then it's down to your skill, but be aware that drilling the rebate for the mortise will be made harder because when you hit the existing hole, your bit will want to move. I'd suggest drilling the holes which meet the cylinder hole edges first, and using an auger/straight style bit (as opposed to a flat bit). That way you have the start of the hole to help keep the bit from wandering.

Obviously the dead-bolt option would be far, far, simpler. Only you can decide which to do.

One thing which is worth considering, however, is the security-level of these locks. Sure, you can get cheap-ass mortise locks, but they're cheap precisely because they are rubbish, probably only 2 or 3 lever, and dead easy to pick. A cylinder dead-bolt will be orders of magnitude more secure, but probably easier to kick-in.

Finally, are you even able to fit a mortise-lock in that position? 'Panelled' doors which have a cross-member morticed into the side-rail should not have mortice lock within 3" or so to avoid weakening the door.

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  • What if I can't find a faceplate for the mortise lockset that covers the entire existing hole? This is my main problem. I can find a mortise lockset with a euro profile lock for R249 ($15), which is still cheaper than any cylindrical lock/deadbolt available around here (They're pretty rare). I'd be willing to tackle the mortise if I can be 100% it won't look like rubbish, but since it's highly likely the faceplate won't cover everything, I'm not sure about it.
    – stan
    Sep 28 '20 at 12:27
  • I took one of the cylindrical knbosets off; the hole is 5.5cm in diameter. My existing mortice covers are 4.2cm wide. I'll need to find one >=5.5.
    – stan
    Sep 28 '20 at 12:41
  • You could repair the hole. If you were to fit the locks in a different place, that might simplify things. You've not got a latch-plate already set-in to the jamb so moving might be the simplest option.
    – SiHa
    Sep 28 '20 at 12:53
  • Thanks for your help. I figured out I can get handles 'on rose' which would work with a mortise lock, but has a handle diameter of 5.4cm (I believe the std size). That would be the best option hiatt-hardware.com/blog/post/…
    – stan
    Sep 28 '20 at 13:00
  • Great, don't forget to check that the centre of the hole for the handle is the same distance from the door-edge as with your cylinder locks.
    – SiHa
    Sep 28 '20 at 13:44

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