Used Nelson for a long time, always had good luck with it. I have used most of their different grades but liked Special forces best then followed by
Yup, measure the OD if the old tube and the new. It could be smaller, it could not.
The only other thing that i can think of, for this problem
Did you ever measure the OD of their pipe?
And just to be clear, as I said before, these fittings were never designed to be used with hardlines....