UNC stands for Unified National Coarse, the inch thread series with the fewest threads per inch for each diameter and the default for general-purpose bolts and tapped holes. Coarse threads start easily, tolerate dirt, plating and handling damage, resist cross-threading and stripping in soft materials such as aluminum and cast iron, and assemble faster. A full callout reads like 1/2-13 UNC-2A: diameter, threads per inch, series and class.
Tapped holes in the casting are all UNC, so don't bring fine-thread cap screws anywhere near this job.
Also heard as
- coarse thread
- UNRC
- USS thread
Do not mix it up with
- UNF: UNF is the fine series of the same diameters, with more threads per inch and a shallower thread.
Related terms
UNF
Unified National Fine: the inch thread series with more threads per inch, such as 1/4-28, 3/8-24 and 1/2-20.
TPI
Threads per inch: the count of thread crests in one inch of length, used to state the pitch of inch threads.
Thread class
A number-and-letter grade, such as 2A or 3B, that sets how much tolerance and allowance an inch thread has.
ISO metric thread
The 60-degree metric screw thread, written as M plus diameter and pitch in millimeters, such as M10 x 1.5.