Originally Posted by sailingdog
The problem with tapping a new hole is that you generally shouldn't use metal fittings in plastic openings... since the metal will generally expand more than the plastic and lead to stress cracks in the plastic.
I would actually say that while the metal expansion is part of the problem even if a plastic nipple were used there is a high chance of stress cracks due to the plastic fitting not being designed with sufficient wall thickness to deal with the tensile (hoop) stress it will encounter when a NPT thread (tapered) is screwed in. The reason when the NPT threads are effective at sealing well is due to the taper applying a force radially outward on the female thread. For this to work well in plastic there needs to be a fairly robust wall thickness to not crack. I doubt that the existing plastic has the required thickness.