^this. I used to work for a fuel/brake line manufacturer for several OEMs. We had hourly QA checks where we had to lay the lines in the gages to check the thread-in and bend angles. They had to thread in easily by hand. If not, we had to make an adjustment to the tube benders. The bend on the line has to be perfectly perpendicular to the fitting or it won't thread in by hand. If it doesn't thread in by hand, the assembly workers at the OEs would reject the pieces.