Q: What is the difference between UNC and UNRC Threads?
A: An “R” pitch can apply to UNRF (unified rounded fine thread) and UNRC (unified rounded coarse thread).
The only difference between UNC threads and UNRC threads (or UNF and UNRF threads) is that the root of the thread – the valley that is between the thread “peaks” – is curved/rounded rather than angular. These roots are rounded with an approximate curvature between 0.108 and 0.144 times the pitch. The UNRC and UNRF makes the roundness between threads mandatory, whereas with the UNC and UNF threads the roundness is optional.
The reason UNRC threads are produced is because this rounded root helps to improve fatigue strength. UNR stands for unified rounded thread.
NOTE: UNC and UNRC threads are compatible with each other, so mixed specifications on fasteners will thread properly.