First off, have to agree with mykeal's
suggestion to revisit these threads. Whenever I work with these, there is always a concern about cross- threading. For that reason, I pay close attention to these areas and If I can't start, by just using my fingers, I back off. I brush and then use air directed at the threads. I also use a product called Electric Motor cleaner, comes in a spray can and has a strong spray. It evaporated very quickly and will not leave any residue. At my final install, I coat the threads with copper based anti-seiz, taking care to leave the vents clear. You will see additional benefits to this anti-seiz when you later reclean.
Good luck and let us know how you make out.
Be Safe !!!