"""""""""""""" I don't have any specs on this at all. The file was included in a 3d printing job. I had to modify a part to be larger which resulted in the screw being too short. """"""""""""""

Since you dont have any of the thread specs and the original prinyt and model were most likely modeled to fit, I woud approach your modification a little different

I would add a cylinder at the ID size of the threads to the part between the head and existing threads. Just need to be sure that the tensile integrity that will be slightly less, is ok with that method...

I will look at your model then attempt to separate the head and threads and make a bridge of 8mm in a poly modeling app.

I'll update later...