I just bought a house from the 1920s and all the interior doors are mortise locks with skeleton keys. I would like to change some from key to privacy with a thumbturn. Is there a way to simply convert or do I have to replace the mortise lock to a privacy lock first? My doors do not have backplates/trim plates and the wife doesn't want to add them to the door, but through my research it doesn't look like I can just buy a thumbturn and insert it. Just want to make sure I'm not missing other options. TIA!
I've done that for a bathroom recently. I replaced the locks of several doors entirely with this lock (fit like a charm). For the thumbturn, I purchased this plate and thumbturn kit. It required a bit of 'macgyvering' but it turned out well.
I had to combine the thumbturn provided in that kit with the key provided by the lock. The thumbturn provides a handle, a square 'arm' with a hole in it, and a pin which connects the thumbturn and the 'arm'. Luckily the shaft of the key fits in the hole of the thumbturn; by cutting off the handle of the key (to appropriate length) and drilling a hole in the shaft you essentially swap the 'arm' provided with a functioning key. Photo illustrates the result:
The pin portion of this thumbturn contraption is hidden behind the plate provided with the thumb turn. Result works and looks great.
You can insert a thumbturn if you see a lock below or above the door latch (when viewing the edge of the door). In this case it would be a matter of drilling a hole to insert the thumbturn through the locking hole inside the door. My advice would be to remove the mortise lock assembly from the door so you can test the locking device before doing anything else.