As a novice, I was wondering how I should find the correct replacement for a mortise lock. I wish to change it to one with a thumbturn deadbolt. This is what it currently looks like and measures:

current door lock

Will I be able to replace it with a lock similar to this:

ideal door lock
source: GJohns.co.uk - Euro Cylinder Sash Lock - Key / Thumbturn

Key (no pun intended) point being the thumbturn deadbolt.

From what I could research on merchant websites, all the euro cylinders seem too long for my door thickness. Case in point the cylinder in the second image measures as such:

ideal door lock cylinder measures
source: GJohns.co.uk - Euro Cylinder Sash Lock - Key / Thumbturn

So, in conclusion,

  • Is there any way I could replace it with the lock above?
  • What would be the ideal cylinder width for my door?
  • 2
    I think the proper term is "thumbturn" not thumbscrew. (I've edited your question accordingly.) Feb 2, 2017 at 2:43
  • @DanielGriscom much appreciated. I was incorrectly using them interchangeably.
    – bPratik
    Feb 2, 2017 at 2:46
  • 1
    I think you meant to refer to the door thickness in comparison to the length of the cylinder unit. I went ahead and edited this correction into your posting.
    – Michael Karas
    Feb 2, 2017 at 6:13
  • Good catch @MichaelKaras! Wonder how I could have been passing through a 5cm wide door! =)
    – bPratik
    Feb 13, 2017 at 10:28
  • 2
    Did you get the lock swapped out? That one could work, but it may have needed another escutcheon for the key side....
    – Jack
    Oct 29, 2017 at 23:36

1 Answer 1


What you've got there is an old-style bit-key mortise lock, which can probably be operated from either side of the door with the same key.

The bad news is that to install a more modern lock the door will probably have to be modified to provide mounting access for the new lock cylinder and thumbturn, and the mortise itself may or may not have to be enlarged (or filled in!) to fit current standard mortise-lock sizes. You may also need a decorative escutcheon plate on each side to cover the old holes.This isn't impossible for a careful DIYer, but a locksmith will have tools and experience to do it faster and more reliably, and I would suggest that it's worth hiring that skill if you care about retaining this door rather than possibly needing to replace it if the job goes wrong.

In fact, if you don't care about retaining this door's appearance and history, simply replacing it with a door already drilled for a cylindrical lockset may be the easiest DIY solution. You might even be able to rescue one that someone else removed.

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