Which statement best describes UNC and UNF thread forms?

• A. UNC is finer than UNF
• B. UNC is coarser than UNF
• C. UNC and UNF are the same
• D. UNF is older than UNC

Answer: (B)

Coarse versus fine threading is determined by pitch—the distance between threads and how many threads fit in an inch. UNC stands for Unified National Coarse and UNF stands for Unified National Fine. Coarse threads have a larger pitch, so there are fewer threads per inch and the threads are deeper. Fine threads have a smaller pitch, resulting in more threads per inch and a shallower profile. Therefore, UNC threads are coarser than UNF threads. This difference affects how easily a bolt engages, with coarse threads typically tightening faster and tolerating dirt or misalignment better, while fine threads offer smoother adjustment and greater resistance to loosening in certain conditions.