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I purchased an older home that has mortise locks in all of the doors. They're all very difficult to use and some of them don't work at all, making the doors difficult to impossible to close. Since I'm going to have to do some work on the doors anyway, I would like to convert the doors to use cylindrical locks, instead.

Is there one standard way to go about doing this? I would guess I'd have to fill the mortise pocket in the door with plywood and then use a template to bore the holes into the door. I'm a little less clear about what would have to be done with the door jamb.

I've never done anything like this, and I want to make sure there isn't anything that I'm missing before attempting to do this.

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Before you consider modifying the door, take off the strike plate, and look in the wall -- on mine, because of how the strike plate was attached and the construction of the wall, there was nothing left to mount the smaller strike plate from a modern lock without a fair bit of work. (in my case, I was replacing the whole door, as I didn't want an all-glass front door) –  Joe Oct 4 '10 at 17:00

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You say you'll be removing the old lock, so why not replace the mortise locks and if you buy 5 or 7 lever ones they'll be more secure than the bored cylindrical lock.

Make sure you buy one with the handle spindle in the same position as the old lock.

When it comes to the door jamb, a lot of mortise locks have a "box keep" rather than just a plate defining the holes for the latch and the lock. See the photo on the Mortise Lock Wikipedia page for an example. This makes them more secure and you don't have to find an exact replacement as you'll be chiselling out a bit more of the jamb anyway.

The problem with replacing the mortise with a bored cylindrical lock is that the new lock is smaller than the old and really has to be in the same location on the door as it's the strongest part of the door. You'd have to fill the mortise hole with a solid block of wood which might well entail digging out more of the door to get a good fit. This would weaken the door. The same applies to the door jamb.

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Agreed -- they tend to be better locks overall, but 100 years of wear and tear might require refurbishing or replacing them. We've got enough older homes in my area that there's still a real locksmith -- luckily, he had the necessary part (well, with a little modification), as the split door spindle we had ordered out of a catalog (restoration hardware? can't remember which one) had the split in the wrong place, and didn't have a groove down the side. –  Joe Oct 4 '10 at 16:55

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