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

1-1/4″ bolt dimensions and screw sizes are mostly used for heavy-duty structural engineering, massive industrial machinery, and high-load mechanical connections. This size have a 1.250″ nominal major diameter and is available in 1-1/4″-7 coarse (UNC), 1-1/4″-12 fine (UNF), or 1-1/4″-18 extra-fine (UNEF) thread pitches, representing 7, 12, and 18 threads per inch (TPI) respectively. 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 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
1-1/4-7 UNC Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 1.2232 1.2478
Pitch Diameter 1.1439 1.155
Minor Diameter Not specified 1.0777
Threads Per Inch (TPI) 7
Major Diameter 1.2314 1.2478
Pitch Diameter 1.1476 1.155
Minor Diameter Not specified 1.0777
Threads Per Inch (TPI) 7
Major Diameter 1.2336 1.25
Pitch Diameter 1.1517 1.1572
Minor Diameter Not specified 1.0799
Threads Per Inch (TPI) 7
1-1/4-8 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 1.2329 1.2479
Pitch Diameter 1.1597 1.1667
Minor Diameter Not specified 1.0991
Threads Per Inch (TPI) 8
Major Diameter 1.235 1.25
Pitch Diameter 1.1635 1.1688
Minor Diameter Not specified 1.1012
Threads Per Inch (TPI) 8
1-1/4-12 UNF Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 1.231 1.2482
Pitch Diameter 1.1849 1.1941
Minor Diameter Not specified 1.149
Threads Per Inch (TPI) 12
Major Diameter 1.2368 1.2482
Pitch Diameter 1.1879 1.1941
Minor Diameter Not specified 1.149
Threads Per Inch (TPI) 12
Major Diameter 1.2386 1.25
Pitch Diameter 1.1913 1.1959
Minor Diameter Not specified 1.1508
Threads Per Inch (TPI) 12
1-1/4-16 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 1.2391 1.2485
Pitch Diameter 1.2028 1.2079
Minor Diameter Not specified 1.1741
Threads Per Inch (TPI) 16
Major Diameter 1.2406 1.25
Pitch Diameter 1.2056 1.2094
Minor Diameter Not specified 1.1756
Threads Per Inch (TPI) 16
1-1/4-18 UNEF Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 1.2398 1.2485
Pitch Diameter 1.2075 1.2124
Minor Diameter Not specified 1.1823
Threads Per Inch (TPI) 18
Major Diameter 1.2413 1.25
Pitch Diameter 1.2103 1.2139
Minor Diameter Not specified 1.1838
Threads Per Inch (TPI) 18
1-1/4-20 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 1.2405 1.2486
Pitch Diameter 1.2114 1.2161
Minor Diameter Not specified 1.1891
Threads Per Inch (TPI) 20
Major Diameter 1.2419 1.25
Pitch Diameter 1.214 1.2175
Minor Diameter Not specified 1.1905
Threads Per Inch (TPI) 20
1-1/4-28 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 1.2423 1.2488
Pitch Diameter 1.2215 1.2256
Minor Diameter Not specified 1.2063
Threads Per Inch (TPI) 28
Major Diameter 1.2435 1.25
Pitch Diameter 1.2237 1.2268
Minor Diameter Not specified 1.2075
Threads Per Inch (TPI) 28
  • 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 1.812 1.875
Width Across Corners 2.066 2.165
Head Height 0.749 0.876
Hex Socket Cap Head – ANSI/ASME B18.3
Dimension Size (in)
MinMax
Head Diameter 1.852 1.875
Head Height 1.236 1.25
Hex Socket Size 7/8 (0.875 in)
Hex Socket Flat Head – ANSI/ASME B18.3
Dimension Size (in)
MinMax
Head Diameter 2.316 2.438
Head Height 0.683 Not specified
Hex Socket Size 7/8 (0.875 in)
Inch to Metric Converter
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Frequently Asked Questions

What is the tap drill size for 1-1/4″-7, 1-1/4″-12, and 1-1/4″-18 screws or bolts?

Tap drill selection is based on the material and required thread engagement. For aluminum, brass, or plastic (75% thread), a 1-7/64″ (1.109″) drill bit is specified for 1-1/4″-7, a 1-11/64″ (1.172″) for 1-1/4″-12, and a 1-3/16″ (1.188″) for 1-1/4″-18. For steel, stainless, or iron (50% thread), a 1-5/32″ (1.156″) drill bit is recommended for 1-1/4″-7, and a 1-13/64″ (1.203″) drill bit is specified for both 1-1/4″-12 and 1-1/4″-18.

What clearance hole size is needed for a 1-1/4″ bolt or screw?

For a Close Fit, a 1-17/64″ (1.266″) drill bit is utilized. For a Normal/Free Fit (standard clearance), a 1-9/32″ (1.281″) drill bit is specified.

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.