1-5/8” Bolt & Screw Dimensions: Diameter & Head Size
1-5/8″ bolt dimensions and sizes are typically used for heavy marine engineering, massive industrial equipment, and high-torque structural assemblies. A standard 1-5/8″ bolt features a 1.625″ nominal major diameter and is available in 1-5/8″-18 extra-fine (UNEF) thread pitch, representing 18 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 large-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.
1-5/8-6 UN Screw & Bolt – ANSI/ASME B1.1
Dimension
Size (in)
Min
Max
Major Diameter
1.6043
1.6225
Pitch Diameter
1.506
1.5142
Minor Diameter
Not specified
1.424
Threads Per Inch (TPI)
6
Major Diameter
1.6068
1.625
Pitch Diameter
1.5105
1.5167
Minor Diameter
Not specified
1.4265
Threads Per Inch (TPI)
6
1-5/8-8 UN Screw & Bolt – ANSI/ASME B1.1
Dimension
Size (in)
Min
Max
Major Diameter
1.6078
1.6228
Pitch Diameter
1.5342
1.5416
Minor Diameter
Not specified
1.474
Threads Per Inch (TPI)
8
Major Diameter
1.61
1.625
Pitch Diameter
1.5382
1.5438
Minor Diameter
Not specified
1.4762
Threads Per Inch (TPI)
8
1-5/8-12 UN Screw & Bolt – ANSI/ASME B1.1
Dimension
Size (in)
Min
Max
Major Diameter
1.6118
1.6232
Pitch Diameter
1.5632
1.5691
Minor Diameter
Not specified
1.524
Threads Per Inch (TPI)
12
Major Diameter
1.6136
1.625
Pitch Diameter
1.5665
1.5709
Minor Diameter
Not specified
1.5258
Threads Per Inch (TPI)
12
1-5/8-16 UN Screw & Bolt – ANSI/ASME B1.1
Dimension
Size (in)
Min
Max
Major Diameter
1.614
1.6234
Pitch Diameter
1.5775
1.5828
Minor Diameter
Not specified
1.549
Threads Per Inch (TPI)
16
Major Diameter
1.6156
1.625
Pitch Diameter
1.5805
1.5844
Minor Diameter
Not specified
1.5506
Threads Per Inch (TPI)
16
1-5/8-18 UNEF Screw & Bolt – ANSI/ASME B1.1
Dimension
Size (in)
Min
Max
Major Diameter
1.6148
1.6235
Pitch Diameter
1.5824
1.5874
Minor Diameter
Not specified
1.5573
Threads Per Inch (TPI)
18
Major Diameter
1.6163
1.625
Pitch Diameter
1.5851
1.5889
Minor Diameter
Not specified
1.5588
Threads Per Inch (TPI)
18
1-5/8-20 UN Screw & Bolt – ANSI/ASME B1.1
Dimension
Size (in)
Min
Max
Major Diameter
1.6155
1.6236
Pitch Diameter
1.5863
1.5911
Minor Diameter
Not specified
1.5641
Threads Per Inch (TPI)
20
Major Diameter
1.6169
1.625
Pitch Diameter
1.5889
1.5925
Minor Diameter
Not specified
1.5655
Threads Per Inch (TPI)
20
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.
What is the tap drill size for a 1-5/8″-18 screw or bolt?
Tap drill selection is based on the material and required thread engagement. For aluminum, brass, or plastic (75% thread), a 1-9/16″ (1.563″) drill bit is specified. For steel, stainless, or iron (50% thread), a 1-37/64″ (1.578″) drill bit is recommended to reduce the load on the tap during the threading process.
What clearance hole size is needed for a 1-5/8″ bolt or screw?
For a Close Fit, a 1-41/64″ (1.641″) drill bit is utilized. For a Normal/Free Fit (standard clearance), a 1-21/32″ (1.656″) drill bit is specified to allow the screw body to pass through the material without binding.
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.
