3-1/4-4 UNC Thread Specs
Standard: ASME B1.1 (UNC)
Find the technical dimensions, threads per inch (TPI), and tapping guidelines for the 3-1/4-4 UNC unified thread.
Compliant with the ASME B1.1 standard, this size has a nominal major diameter of 3.25 inches (82.55 mm) with a pitch of 0.25 inches (TPI of 4).
For general engineering or fastening, the standard recommended tap drill is 3 inches (76.2 mm) to achieve adequate thread engagement.
Dimensional Specifications (Inches & mm)
Size Designation
3-1/4-4 UNC
Major Diameter (Nominal)
3.25 in(82.55 mm)
TPI (Threads per Inch)
4
Pitch
0.25 in
Pitch Diameter
3.0876 in(78.43 mm)
Minor Diameter (Ext)
2.9794 in(75.68 mm)
Minor Diameter (Int)
3.0064 in(76.36 mm)
Thread Height
0.2165 in
Drilling & Strength Specs
Tap Drill Size
3 in(76.2 mm)
Stress Area
7.0989 in²(4579.92 mm²)
Quick Unit Converter
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Frequently Asked Questions
What is the tap drill size for 3-1/4-4 UNC?
The recommended tap drill size for a 3-1/4-4 UNC thread is 3 inches (approx. 76.2 mm) to achieve standard thread engagement.
How many threads per inch (TPI) does a 3-1/4-4 UNC bolt have?
A standard 3-1/4-4 UNC thread has exactly 4 threads per inch (TPI), with a corresponding linear pitch of 0.25 inches.
What is the major diameter of a 3-1/4-4 UNC thread in millimeters?
The nominal major diameter of a 3-1/4-4 UNC thread is 3.25 inches, which converts to 82.55 mm.