Transplanting the innards of a Yale (brand) nightlatch

1 Sep 2014
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United Kingdom
Short question: does anyone know how easy it is to transplant the innards of a Yale 91 nightlatch (link):
into the Yale 89 nightlatch shown in the picture below:

Long explanation: The Yale 89 nightlatch (in photo) is on the front door of an old house which has been converted into flats. Behind the door are 3 self-contained flats each with their own, securely lockable, front door. Once upon a time there was an entryphone type lock, but someone forced their way past it and it was replaced (as above); at the same time a mortice lock was installed which has, on the inside, a knob to (un)lock. The Yale 89, using the snib, can be deadlocked from the inside so you can't use a key externally, equally it can be deadlocked, from the outside, by turning the key 360 degrees "the wrong way" which stops anyone opening the door from the inside (not too good in a fire). I'd like to retain the shiny look partly 'cos the staple is so well embedded in the wall that I'd make a right mess of changing that to a DMG (Dark Metal Grey) one which is the only colour available for a Yale 91.

I'm prepared to take the risk that moving the innards from a 91 to an 89 might work but not the other way round. If anyone's got any knowledge or hints (just thinking out loud - circlip pliers needed ?) then I'd be most grateful.
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In case this is helpful to anyone else.
Swapping over the bits to prevent deadlocking from the outside (using the key) was easy. But I couldn't do anything about the snib; there was no obvious way of removing the snib button from the snib slider (inside the "shell"). Also, in this case, there'd been a redesign of the snib workings between the (> 20 years ?) old Yale 89 and the new Yale 91.
Anyway, I'm a happy bunny that people can't be locked in any longer.

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