2-1/2-4 UNC Thread Specs
Standard: ASME B1.1 (UNC)
Find the technical dimensions, threads per inch (TPI), and tapping guidelines for the 2-1/2-4 UNC unified thread.
Compliant with the ASME B1.1 standard, this size has a nominal major diameter of 2.5 inches (63.5 mm) with a pitch of 0.25 inches (TPI of 4).
For general engineering or fastening, the standard recommended tap drill is 2.25 inches (57.15 mm) to achieve adequate thread engagement.
Dimensional Specifications (Inches & mm)
Size Designation
2-1/2-4 UNC
Major Diameter (Nominal)
2.5 in(63.5 mm)
TPI (Threads per Inch)
4
Pitch
0.25 in
Pitch Diameter
2.3376 in(59.38 mm)
Minor Diameter (Ext)
2.2294 in(56.63 mm)
Minor Diameter (Int)
2.2564 in(57.31 mm)
Thread Height
0.2165 in
Drilling & Strength Specs
Tap Drill Size
2.25 in(57.15 mm)
Stress Area
3.9988 in²(2579.88 mm²)
Quick Unit Converter
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Frequently Asked Questions
What is the tap drill size for 2-1/2-4 UNC?
The recommended tap drill size for a 2-1/2-4 UNC thread is 2.25 inches (approx. 57.15 mm) to achieve standard thread engagement.
How many threads per inch (TPI) does a 2-1/2-4 UNC bolt have?
A standard 2-1/2-4 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 2-1/2-4 UNC thread in millimeters?
The nominal major diameter of a 2-1/2-4 UNC thread is 2.5 inches, which converts to 63.5 mm.