1-3/8” Bolt & Screw Dimensions: Diameter & Head Size

1-3/8″ bolt dimensions and screw sizes are vital for heavy-duty industrial anchoring, massive mechanical assemblies, and high-strength structural engineering. This size have a 1.375″ nominal major diameter and is available in 1-3/8″-6 coarse (UNC), 1-3/8″-12 fine (UNF), or 1-3/8″-18 extra-fine (UNEF) thread pitches, representing 6, 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
1-3/8-6 UNC Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 1.3453 1.3726
Pitch Diameter 1.2523 1.2643
Minor Diameter Not specified 1.1741
Threads Per Inch (TPI) 6
Major Diameter 1.3544 1.3726
Pitch Diameter 1.2563 1.2643
Minor Diameter Not specified 1.1741
Threads Per Inch (TPI) 6
Major Diameter 1.3568 1.375
Pitch Diameter 1.2607 1.2667
Minor Diameter Not specified 1.1765
Threads Per Inch (TPI) 6
1-3/8-8 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 1.3578 1.3728
Pitch Diameter 1.2844 1.2916
Minor Diameter Not specified 1.224
Threads Per Inch (TPI) 8
Major Diameter 1.36 1.375
Pitch Diameter 1.2884 1.2938
Minor Diameter Not specified 1.2262
Threads Per Inch (TPI) 8
1-3/8-12 UNF Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 1.3559 1.3731
Pitch Diameter 1.3096 1.319
Minor Diameter Not specified 1.2739
Threads Per Inch (TPI) 12
Major Diameter 1.3617 1.3731
Pitch Diameter 1.3127 1.319
Minor Diameter Not specified 1.2739
Threads Per Inch (TPI) 12
Major Diameter 1.3636 1.375
Pitch Diameter 1.3162 1.3209
Minor Diameter Not specified 1.2758
Threads Per Inch (TPI) 12
1-3/8-16 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 1.3641 1.3735
Pitch Diameter 1.3277 1.3329
Minor Diameter Not specified 1.2991
Threads Per Inch (TPI) 16
Major Diameter 1.3656 1.375
Pitch Diameter 1.3305 1.3344
Minor Diameter Not specified 1.3006
Threads Per Inch (TPI) 16
1-3/8-18 UNEF Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 1.3648 1.3735
Pitch Diameter 1.3325 1.3374
Minor Diameter Not specified 1.3073
Threads Per Inch (TPI) 18
Major Diameter 1.3663 1.375
Pitch Diameter 1.3352 1.3389
Minor Diameter Not specified 1.3088
Threads Per Inch (TPI) 18
1-3/8-20 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 1.3655 1.3736
Pitch Diameter 1.3364 1.3411
Minor Diameter Not specified 1.3141
Threads Per Inch (TPI) 20
Major Diameter 1.3669 1.375
Pitch Diameter 1.339 1.3425
Minor Diameter Not specified 1.3155
Threads Per Inch (TPI) 20
1-3/8-28 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 1.3673 1.3738
Pitch Diameter 1.3465 1.3506
Minor Diameter Not specified 1.3313
Threads Per Inch (TPI) 28
Major Diameter 1.3685 1.375
Pitch Diameter 1.3487 1.3518
Minor Diameter Not specified 1.3325
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 1.994 2.062
Width Across Corners 2.273 2.382
Head Height 0.81 0.94
Hex Socket Cap Head – ANSI/ASME B18.3
Dimension Size (in)
MinMax
Head Diameter 2.038 2.062
Head Height 1.36 1.375
Hex Socket Size 1 (1 in)
Hex Socket Flat Head – ANSI/ASME B18.3
Dimension Size (in)
MinMax
Head Diameter 2.553 2.688
Head Height 0.755 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 1-3/8″-6, 1-3/8″-12, and 1-3/8″-18 screws or bolts?

Tap drill selection is based on the material and required thread engagement. For aluminum, brass, or plastic (75% thread), a 1-7/32″ (1.219″) drill bit is specified for 1-3/8″-6, a 1-19/64″ (1.297″) for 1-3/8″-12, and a 1-5/16″ (1.313″) for 1-3/8″-18. For steel, stainless, or iron (50% thread), a 1-17/64″ (1.266″) drill bit is recommended for 1-3/8″-6, and a 1-21/64″ (1.328″) drill bit is specified for both 1-3/8″-12 and 1-3/8″-18.

What clearance hole size is needed for a 1-3/8″ bolt or screw?

For a Close Fit, a 1-25/64″ (1.391″) drill bit is utilized. For a Normal/Free Fit (standard clearance), a 1-13/32″ (1.406″) 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.