Use couplings. So what if crap gets in the threads, you boil for at least an hour anyway so no chance of infection there. If getting the threads inside a coupler clean is a major concern then take a nipple and cut through the threads perpendicular, then use this nipple as a thread chaser. It will scrape out anything inside the coupler. I don't really think going that far is necessary though. Exterior threads are easier to damage and harder to fix once you have them welded to the inside of your kettle(tap vs die, taps take less room to use). I hope this helps.