Replacing a night latch cylinder is for the most part a very easy process and there are lots of videos on YouTube demonstrating how this is done, however frequently there is a bit more too it. Perhaps the lock has been badly fitted to start with, or been badly repaired at some stage.
There are two videos in this post, one which goes through replacing a night latch cylinder where the installation is straightforward and another which demonstrates what to do when the installation or subsequent repairs has be botched.
You can view a short extract from the video below
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