DIAMETER VERSUS NUMBER OF THREADS PER INCH

Coarse
Fine
Coarse
Fine
00-90
00-96
12-24
12-28
----
0-80
1/4-20
1/4-28
1-64
1-72
5/16-18
5/16-24
2-56
2-64
3/8-16
3/8-24
3-48
3-56
7/16-14
7/16-20
4-40
4-48
1/2-13
1/2-20
5-40
5-48
9/16-12
9/16-18
6-32
6-40
5/8-11
5/8-18
8-32
8-36
3/4-10
3/4-16
10-24
10-32
7/8-9
7/8-14
1-8
1-14

Thread Pitch

UNC The Unified national Coarse Thread was introduced in the mid nineteenth century by Whitmore and is still used today. In fact, within the past twenty years there has been a popularity shift to UNC, which analysts say is due to the need to simplify the diameter/pitch group combinations.
UNF The Unified National Fine thread is commonly used for high strength applications, having less thread depth and larger minor diameters than UNC threads. Less wall thickness is needed for internal threads. External threads of the UNF series are stronger in tension and have higher torsional and transverse shear strengths. One disadvantage is that UNF threads have a tendency to loosen due to their smaller lead angle.
UNR Unified National threads require that the root of the threads have a radius. This call-out was used more in the past since today all fasteners classified as Unified National threads are manufactured with a root radius if they are smaller than one inch in diameter. Use UNR callout only on diameters larger than one inch.
UNEF/ UNREF The Unified National Extra Fine series is used for threaded parts and equipment requiring fine adjustments and for thin wall nuts and tubes.
UNJ Unified National "J" formed threads are characteristically known by the controlled root radius and increased minor diameter. They offer superior fatigue strength without sacrificing static strength. Thread "fit" is 3A and 3B used for critical applications such as aerospace.

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