Refining NPT plug simulations sounded like a software problem. It turned out to be a metrology problem.
Modeling thread geometry directly is expensive and rarely necessary if you can validate against a precision test tube and a CMM. By integrating real measurements back into the FEA workflow, we ended up with a faster simulation that we actually trusted.
The general lesson: any time a simulation surprises you, walk to the metrology lab before you change the mesh.