Personally, I think AICC should be punted over the goal post of so long compliance standards. and some vendors like to offer multiple choices, and yes AICC does different things, but so does driving a moped and I rarely seen people cruising down the interstate on one (unless you live in LA).
Part One: How to Measure Thread Size. 1  In general, the thread should match the fabric in size and weight. For heavier, thicker fabrics, use heavier thread. 2  Heavier threads create more visible stitches. 3  V-69 is the heaviest size recommended for sewing machines. 4  Heavier threads are stronger.
The fitting they will show you will likely be a 1/4 NPT. You will look at the diameter of the thread and say to yourself, this ain't no 1/4 fitting. It will be. The actual diameter of the 1/4 NPT thread will be just over 1/2. The size of an NPT fitting, teh 1/4, the 3/8, 1/2 and so on are reference sizes.