The 40 signifies the 'backset, the face of the lock back to the centre line of the cut out for the barrel, 92 is the 'PZ', the distance between centre lines of the handle spindle and the circular portion of the barrel cut out.
Searching for 40mm backset GU gearbox may yield better results, you may have to upgrade it to a MK2, the GU was notorious for failures
No I don't, just do a search and you'll see plenty of results, you'll even see pictures of the newer mk2 GU gearbox, you shouldn't need to replace the hooks as the GU gearbox can be separated from the main strip by removing 2 or 3 little torx screws
Will the Mk2 style GU gearboxes fit as straight replacements for the old style ones?
I need to replace a broken 40/92 old style gearbox. It seems the old style ones are now only available in a 30mm or 35mm backset but I definitely need 40mm.
If possible I want to keep the other existing bits (strip etc.) and I want to avoid having to rout the wooden door.
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