A reducing bushing is a NPT pipe nipple that has a male end (the end you would thread into the radiator neck) and a female end. You don't need the female thread for anything in this application. It's just that because of its intended normal use (reducing 1 pipe size to another) the construction is such that there are flats on the female end that a pipe wrench can grab. These flats make a nice "lip" for the radiator hose to slip over providing additional grip. Works great. Go to a hardware store (like Sears) and you'll find them in all sizes. You'll get the idea when you see it. I forget the exact size that will thread into the rad neck but its a good tight fit.