7/8” Bolt & Screw Dimensions: Diameter & Head Size

7/8″ bolt dimensions and screw sizes are mostly used for heavy industrial fasteners, bridge construction, and large-scale mechanical engineering. This size have a 0.875″ nominal major diameter and is available in 7/8″-9 coarse (UNC), 7/8″-14 fine (UNF), or 7/8″-20 extra-fine (UNEF) thread pitches, representing 9, 14, and 20 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
7/8-9 UNC Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 0.8523 0.8731
Pitch Diameter 0.7914 0.8009
Minor Diameter Not specified 0.7408
Threads Per Inch (TPI) 9
Major Diameter 0.8592 0.8731
Pitch Diameter 0.7946 0.8009
Minor Diameter Not specified 0.7408
Threads Per Inch (TPI) 9
Major Diameter 0.8611 0.875
Pitch Diameter 0.7981 0.8028
Minor Diameter Not specified 0.7427
Threads Per Inch (TPI) 9
7/8-12 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 0.8619 0.8733
Pitch Diameter 0.8136 0.8192
Minor Diameter Not specified 0.7741
Threads Per Inch (TPI) 12
Major Diameter 0.8636 0.875
Pitch Diameter 0.8167 0.8209
Minor Diameter Not specified 0.7758
Threads Per Inch (TPI) 12
7/8-14 UNF Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 0.8579 0.8734
Pitch Diameter 0.8189 0.827
Minor Diameter Not specified 0.7883
Threads Per Inch (TPI) 14
Major Diameter 0.8631 0.8734
Pitch Diameter 0.8216 0.827
Minor Diameter Not specified 0.7883
Threads Per Inch (TPI) 14
Major Diameter 0.8647 0.875
Pitch Diameter 0.8245 0.8286
Minor Diameter Not specified 0.7899
Threads Per Inch (TPI) 14
7/8-16 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 0.8641 0.8735
Pitch Diameter 0.828 0.8329
Minor Diameter Not specified 0.7991
Threads Per Inch (TPI) 16
Major Diameter 0.8656 0.875
Pitch Diameter 0.8307 0.8344
Minor Diameter Not specified 0.8006
Threads Per Inch (TPI) 16
7/8-20 UNEF Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 0.8656 0.8737
Pitch Diameter 0.8367 0.8412
Minor Diameter Not specified 0.8142
Threads Per Inch (TPI) 20
Major Diameter 0.8669 0.875
Pitch Diameter 0.8391 0.8425
Minor Diameter Not specified 0.8155
Threads Per Inch (TPI) 20
7/8-28 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 0.8673 0.8738
Pitch Diameter 0.8467 0.8506
Minor Diameter Not specified 0.8313
Threads Per Inch (TPI) 28
Major Diameter 0.8685 0.875
Pitch Diameter 0.8489 0.8518
Minor Diameter Not specified 0.8325
Threads Per Inch (TPI) 28
7/8-32 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 0.8679 0.8739
Pitch Diameter 0.8499 0.8536
Minor Diameter Not specified 0.8367
Threads Per Inch (TPI) 32
Major Diameter 0.869 0.875
Pitch Diameter 0.8519 0.8547
Minor Diameter Not specified 0.8378
Threads Per Inch (TPI) 32
  • 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 1.269 1.312
Width Across Corners 1.447 1.516
Head Height 0.531 0.604
Hex Socket Cap Head – ANSI/ASME B18.3
Dimension Size (in)
MinMax
Head Diameter 1.291 1.312
Head Height 0.864 0.875
Hex Socket Size 3/4 (0.75 in)
Hex Socket Flat Head – ANSI/ASME B18.3
Dimension Size (in)
MinMax
Head Diameter 1.604 1.688
Head Height 0.468 Not specified
Hex Socket Size 9/16 (0.562 in)
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Frequently Asked Questions

What is the tap drill size for 7/8″-9, 7/8″-14, and 7/8″-20 screws or bolts?

The appropriate tap drill depends on the material and required thread engagement. For aluminum, brass, or plastic (75% thread), a 49/64″ (0.766″) drill bit is specified for 7/8″-9, a 13/16″ (0.813″) for 7/8″-14, and a 53/64″ (0.828″) for 7/8″-20. For steel, stainless, or iron (50% thread), a 51/64″ (0.797″) drill bit is recommended for 7/8″-9, a 53/64″ (0.828″) for 7/8″-14, and a 27/32″ (0.844″) for 7/8″-20.

What clearance hole size is needed for a 7/8″ bolt or screw?

For a Close Fit, a 57/64″ (0.891″) drill bit is utilized. For a Normal/Free Fit (standard clearance), a 29/32″ (0.906″) 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.