Sash lock size

[Flinstone]

What is the rule of thumb when selecting a lock size to fit to a new external door?

The door timber thickness is 44mm and 93mm deep. I purchased a 78mm deep lock based on what I saw on the old door but now feel that there wouldn't seem much material left after cutting the mortice out.

Would it be wiser to exchange it for a 65mm lock?

Any comments please?

 

[HandyHands]

Depends on the depth of the frame when the door is closed - will a 2.5" lock have sufficient clearance for handles/keyhole, and still be usable without bruising your knuckles everytime the door is opened.

If you keep the mortice tight, you shouldn't have a problem.