5/16-18 UNC Thread Specs
Standard: ASME B1.1 (UNC)
Find the technical dimensions, threads per inch (TPI), and tapping guidelines for the 5/16-18 UNC unified thread.
Compliant with the ASME B1.1 standard, this size has a nominal major diameter of 0.3125 inches (7.94 mm) with a pitch of 0.05556 inches (TPI of 18).
For general engineering or fastening, the standard recommended tap drill is 0.2569 inches (6.53 mm) to achieve adequate thread engagement.
Dimensional Specifications (Inches & mm)
Size Designation
5/16-18 UNC
Major Diameter (Nominal)
0.3125 in(7.94 mm)
TPI (Threads per Inch)
18
Pitch
0.05556 in
Pitch Diameter
0.2764 in(7.02 mm)
Minor Diameter (Ext)
0.2524 in(6.41 mm)
Minor Diameter (Int)
0.2584 in(6.56 mm)
Thread Height
0.0481 in
Drilling & Strength Specs
Tap Drill Size
0.2569 in(6.53 mm)
Stress Area
0.0524 in²(33.83 mm²)
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Frequently Asked Questions
What is the tap drill size for 5/16-18 UNC?
The recommended tap drill size for a 5/16-18 UNC thread is 0.2569 inches (approx. 6.53 mm) to achieve standard thread engagement.
How many threads per inch (TPI) does a 5/16-18 UNC bolt have?
A standard 5/16-18 UNC thread has exactly 18 threads per inch (TPI), with a corresponding linear pitch of 0.05556 inches.
What is the major diameter of a 5/16-18 UNC thread in millimeters?
The nominal major diameter of a 5/16-18 UNC thread is 0.3125 inches, which converts to 7.94 mm.