Hi we have been making functional parts by FDM for nearly a year for our prototypes and pre-production runs.

In practice using nylon and ABS we have been able to print internal threads down to M3 but it has taken some experimenting with hole expansion in the slicer just sometimes we have to clear the thread form out with a tap.

We have printed female BSP(like NPT)1/8" threads and hydraulic tested them to 10 Bar in Nylon6 LW with no problems once we had adjusted hole expansion correctly.

Some times when overhangs cause difficulties we make the item in 2 parts out of ABS and solvent weld them together.

The great advantage of making small parts bf FDM is the cost of each part is low