1-3/8-18 UNEF Thread Specs
Standard: ASME B1.1 (UNEF)
Find the technical dimensions, threads per inch (TPI), and tapping guidelines for the 1-3/8-18 UNEF unified thread.
Compliant with the ASME B1.1 standard, this size has a nominal major diameter of 1.375 inches (34.92 mm) with a pitch of 0.05556 inches (TPI of 18).
For general engineering or fastening, the standard recommended tap drill is 1.3194 inches (33.51 mm) to achieve adequate thread engagement.
Dimensional Specifications (Inches & mm)
Size Designation
1-3/8-18 UNEF
Major Diameter (Nominal)
1.375 in(34.92 mm)
TPI (Threads per Inch)
18
Pitch
0.05556 in
Pitch Diameter
1.3389 in(34.01 mm)
Minor Diameter (Ext)
1.3149 in(33.4 mm)
Minor Diameter (Int)
1.3209 in(33.55 mm)
Thread Height
0.0481 in
Drilling & Strength Specs
Tap Drill Size
1.3194 in(33.51 mm)
Stress Area
1.3703 in²(884.05 mm²)
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Frequently Asked Questions
What is the tap drill size for 1-3/8-18 UNEF?
The recommended tap drill size for a 1-3/8-18 UNEF thread is 1.3194 inches (approx. 33.51 mm) to achieve standard thread engagement.
How many threads per inch (TPI) does a 1-3/8-18 UNEF bolt have?
A standard 1-3/8-18 UNEF thread has exactly 18 threads per inch (TPI), with a corresponding linear pitch of 0.05556 inches.
What is the major diameter of a 1-3/8-18 UNEF thread in millimeters?
The nominal major diameter of a 1-3/8-18 UNEF thread is 1.375 inches, which converts to 34.92 mm.