tap sizes
DFW is correct, and I would like to add my own thoughts. Most inch cutting taps from #10 to 1/2" seam to be sold having a H3 limit. But other limits are available. Form taps use a higher limit, I presume because the material will spring back after forming. With my 1/4-20 taps for example, cutting are H3, and roll are H8.
Machinery's Handbook will give the max & min pitch diameter for a given internal thread. The pitch diameter is determined by the H limit on the tap. A tap with a H3 limit, for example, might give a different pitch diameter in different materials. I find that many plastics shrink, so you might need a higher H limit to get the 2B specs. How much difference is determined by trial & error.
Sometimes in production tapping, you start off with the highest H limit that meets specs, and as the tap wears, you will still get good holes for a while.
Metric taps use D limits, but the concept is the same.
Balax & OSG have excellent technical sections in their catalogs.