3-1/4” Bolt & Screw Dimensions: Diameter & Head Size

3-1/4″ bolt dimensions and screw sizes are specialized for massive turbine housings, maritime propulsion systems, and heavy-duty structural foundation anchors. This size have a 3.250″ nominal major diameter and is available in 3-1/4″-4 coarse (UNC) thread pitch, representing 4 threads per inch (TPI). This reference chart follows ANSI/ASME B18.6.3 and B18.3 standards for the head size and ANSI/ASME B1.1 standards for the thread dimensions to ensure compatibility across extreme-scale imperial fastening applications.

Disclaimer: Dimensions and tolerances may vary slightly depending on head style, manufacturer, or material — always verify with the official product datasheet before use.

Thread diagram fot bolt and screw depicting the major diameter, minor diameter, pitch diameter, and threads per inch
Hex head diagram
Cap head diagram
Low-cap head diagram
Button head diagram
Flat head diagram
Pan head diagram
3-1/4-4 UNC Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 3.211 3.2467
Pitch Diameter 3.068 3.0843
Minor Diameter Not specified 2.949
Threads Per Inch (TPI) 4
Major Diameter 3.2229 3.2467
Pitch Diameter 3.0734 3.0843
Minor Diameter Not specified 2.949
Threads Per Inch (TPI) 4
Major Diameter 3.2262 3.25
Pitch Diameter 3.0794 3.0876
Minor Diameter Not specified 2.9523
Threads Per Inch (TPI) 4
3-1/4-6 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 3.229 3.2472
Pitch Diameter 3.1294 3.1389
Minor Diameter Not specified 3.0487
Threads Per Inch (TPI) 6
Major Diameter 3.2318 3.25
Pitch Diameter 3.1346 3.1417
Minor Diameter Not specified 3.0515
Threads Per Inch (TPI) 6
3-1/4-8 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 3.2324 3.2474
Pitch Diameter 3.1575 3.1662
Minor Diameter Not specified 3.0986
Threads Per Inch (TPI) 8
Major Diameter 3.235 3.25
Pitch Diameter 3.1623 3.1688
Minor Diameter Not specified 3.1012
Threads Per Inch (TPI) 8
3-1/4-12 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 3.2367 3.2481
Pitch Diameter 3.1876 3.194
Minor Diameter Not specified 3.1489
Threads Per Inch (TPI) 12
Major Diameter 3.2386 3.25
Pitch Diameter 3.1911 3.1959
Minor Diameter Not specified 3.1508
Threads Per Inch (TPI) 12
3-1/4-16 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 3.2389 3.2483
Pitch Diameter 3.202 3.2077
Minor Diameter Not specified 3.1739
Threads Per Inch (TPI) 16
Major Diameter 3.2406 3.25
Pitch Diameter 3.2051 3.2094
Minor Diameter Not specified 3.1756
Threads Per Inch (TPI) 16
  • Major Diameter – The largest diameter of the external thread, measured crest to crest. This defines the nominal size of the screw or bolt.
  • Pitch Diameter – The diameter of an imaginary cylinder where the thread thickness equals the space between threads. This dimension primarily controls thread fit and tolerance class of 2A and 3A.
  • Minor Diameter – The smallest diameter of the external thread, measured root to root. This affects thread strength and clearance.
  • Threads Per Inch (TPI) – The number of complete threads within one inch of thread length. Higher TPI values indicate a finer thread pitch.
Hex Head – ANSI/ASME B18.2.1
Dimension Size (in)
MinMax
Width Across Flats 4.712 4.875
Width Across Corners 5.372 5.629
Head Height 1.936 2.251
Hex Socket Cap Head – ANSI/ASME B18.3
Dimension Size (in)
MinMax
Head Diameter 4.837 4.875
Head Height 3.228 3.25
Hex Socket Size 2-1/4 (2.25 in)
Inch to Metric Converter
Enter inches to convert to mm
Previous & Next Size Chart
← 3
→ 3-1/2

← Choose Another Size

Frequently Asked Questions

What is the difference between a bolt and a screw?

The difference is based on the intent of the fastener’s use; If the primary intent is for the fastener to pass through an unthreaded holes and be assembled with a nut, it is a Bolt. In this application, you usually hold the head and turn the nut to apply tension. But if the primary intent is for the fastener to be installed into a threaded (tapped) hole, it is a Screw. In this application, the connection is tightened by turning the head of the fastener itself.