David, these taps are NPT, or National Pipe Taper, and are designed for tapered pipe threads.. What I believe you are asking for is a tap to build or chase the threads on a nut, which I am assuming is to be screwed on a bolt. If so, you are needing either a 3/8"-16 SAE tap for course thread or a 3/8"-24 SAE tap for fine thread. The SAE is a straight thread.used on bolts and stands for Society of Automotive Engineering. NPT and SAE taps are completely different in design and size.
Hopefully that helps!