UNF stands for Unified National Fine. For the same diameter it has a shallower thread and a tensile stress area roughly 10 to 15 percent larger than UNC, giving more strength and finer adjustment per turn. Fine threads suit thin walls, adjusting screws and high-strength bolting in automotive and aerospace work, but they gall and cross-thread more easily and need a stronger internal thread to avoid stripping.
Caliper bracket bolts are 7/16-20 UNF, so check the bin label twice before you run them in.
Plenty of mechanics call fine thread SAE and coarse thread USS, and plenty of parts bins are labeled that way.
Also heard as
- fine thread
- UNRF
- SAE thread
Do not mix it up with
- UNC: UNC is the coarse series: fewer, deeper threads that start more easily and tolerate soft materials better.
Related terms
UNC
Unified National Coarse: the standard coarse inch thread series, such as 1/4-20, 3/8-16 and 1/2-13.
TPI
Threads per inch: the count of thread crests in one inch of length, used to state the pitch of inch threads.
Galling
Adhesive wear where thread surfaces cold-weld and tear under load, seizing a nut partway down a bolt.
Cross-thread
To start a nut or screw crooked so its thread cuts across the mating thread instead of following it.