Page 19 - Threading_Handbook

Tolerance System UN/ UNC/ UNJ/ UNR/ UNS/ UNRS/ UNF/ UNEF Standards
The UN/ UNJ/ UNR/ UNS/ UNRS/ UNF/ UNEF standards define 3 classes of accuracy for
external threads and internal threads:

External Threading Internal Threading INTRODUCTION
• 3A (Tight tolerance) • 3B (Tight tolerance)

• 2A (Medium tolerance) • 2B (Medium tolerance)
• 1A (Loose tolerance) • 1B (Loose tolerance)

** bold - most common
The value and range of the tolerance according to accuracy class are described by tables,
formulas, and charts in the UN standard.
Possible combinations are described in the UN standard.
Not all combinations exist in all three tolerance classes.
Examples for thread description according to UN/ UNC/ UNJ/ UNR/ UNS/ UNRS/ UNF/
UNEF standards with accuracy class:

Major Thread per Inch
Diameter (TPI) Thread Standard - Accuracy Class

External Threading Internal Threading

3/8 16 UNC – 3A 3/8 16 UNC – 3B
1/2 20 UNF – 2A 1/2 20 UNF – 2B
9/16 24 UNEF – 1A 9/16 24 UNEF – 1B

Useful information

• UN standard for external threading can be connected with UNJ and UN standards
for internal threading.

• UNR standard for external threading can be connected with UNJ and UN standards
for internal threading.
• UNJ standard for external threading can be connected with UNJ standard for internal
threading only.
• UN standard for internal threading cannot be connected with UNJ standard
for external threading.

• UNJ standard for internal threading can be connected with UN/ UNC/ UNJ/ UNR/ UNS/
UNRS/ UNF/ UNEF standards for external threading.

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