What class of threads are interference fit threads available in the older American National Coarse (NC) series?

• A. Class 5
• B. Class 1
• C. Class 2
• D. Class 3

Answer: (A)

Interference fits arise when the tolerance zones for the two mating threads overlap so much that the external thread can be larger than the internal thread, requiring deformation or forcing to assemble. In the older American National Coarse (NC) series, that condition is only achieved in the higher-tolerance class. Tightening the tolerances in that class shifts the external thread’s usable size so its maximum can exceed the internal thread’s minimum, creating interference during assembly. The other NC classes are designed for clearance or near-clearance fits, so they won’t produce interference. Therefore, the high-tolerance NC class is the one that provides interference fits.