13/16 - 28 28-UN - Basic Thread Dimensions

Property Inch mm
Nominal Diameter 0.812 20.625
Pitch (TPI)28
Pitch (Distance) 0.0357 0.907
Pitch Diameter 0.7888 20.036
Minor Diameter 0.7695 19.545
Thread Height 0.0193 0.49
Addendum 0.0116 0.295
Crest (External)
Root (Internal)
0.0045 0.114
Tap Drill Size
(75% depth)
0.777 19.74
Lead Angle
(Single Start)
0.83°
Thread Family Unified Constant Pitch 28 TPI (28-UN)
Relevant Standards ASME B1.1-2003
Equivalent Metric ThreadM20 X 1

For more threading data scroll down, or use the Thread Calculator

Table of Content


13/16 - 28 28-UN Possible Thread Classes

External Threads

Choose tolerance class: 2A, 3A

Metric ISO External Thread - Tolerance sketch

Internal Threads

Choose tolerance class: 2B, 3B

Metric ISO Internal Thread - Tolerance sketch

13/16 - 28 28-UN 2A (External)

* Permisable values for thread Class 2A

# Description Value (Inch) Value (mm)
es Allowance 0.0011 (Ref) 0.028 (Ref)
d Major Diameter 0.8044 - 0.8109 20.432 - 20.597
d2 Pitch Diameter 0.784 - 0.7877 19.914 - 20.008
d3 Minor Diameter
(UNR Only)
0.7684 (Ref) 19.517 (Ref)
α Varation of 30° anngle (+/-) 1.3333°
P Varation of pitch (+/-) 0.0011 (Ref) 0.028 (Ref)

* Values are calculated for Length of engagement of 5xPitch. For calculations of different engagement length use the Thread Calculator

* Back to Thread Class Selection


13/16 - 28 28-UN 3A (External)

* Permisable values for thread Class 3A

# Description Value (Inch) Value (mm)
es Allowance 0 (Ref) 0 (Ref)
d Major Diameter 0.8055 - 0.812 20.46 - 20.625
d2 Pitch Diameter 0.7861 - 0.7888 19.967 - 20.036
d3 Minor Diameter
(UNR Only)
0.7695 (Ref) 19.545 (Ref)
α Varation of 30° anngle (+/-) 1.3333°
P Varation of pitch (+/-) 0.0008 (Ref) 0.02 (Ref)

* Values are calculated for Length of engagement of 5xPitch. For calculations of different engagement length use the Thread Calculator

* Back to Thread Class Selection


13/16 - 28 28-UN 2B (Internal)

* Permisable values for thread Class 2B

# Description Value (Inch) Value (mm)
D1 Minor Diameter 0.773 - 0.782 19.634 - 19.863
D2 Pitch Diameter 0.7888 - 0.7936 20.036 - 20.157
D Major Diameter 0.8120 (Min) 20.625 (Min)
α Varation of 30° anngle (+/-) 1.3333°
P Varation of Pitch (+/-) 0.0014 (Ref) 0.036 (Ref)

* Values are calculated for Length of engagement of 5xPitch. For calculations of different engagement length use the Thread Calculator

* Back to Thread Class Selection


13/16 - 28 28-UN 3B (Internal)

* Permisable values for thread Class 3B

# Description Value (Inch) Value (mm)
D1 Minor Diameter 0.7733 - 0.7796 19.642 - 19.802
D2 Pitch Diameter 0.7888 - 0.7924 20.036 - 20.127
D Major Diameter 0.8120 (Min) 20.625 (Min)
α Varation of 30° anngle (+/-) 1.3333°
P Varation of Pitch (+/-) 0.001 (Ref) 0.025 (Ref)

* Values are calculated for Length of engagement of 5xPitch. For calculations of different engagement length use the Thread Calculator

* Back to Thread Class Selection


Tap Drill Sizes for 13/16 - 28 28-UN

  • If not specified, assume a thread depth of 75%.
  • You can learn more about thread depth in tapping here.
  • To get the tap drill size in other depths, use our Tap Drill Size Calculator .
Thread Depth Tap Drill Size (Inch) Tap Drill Size (mm)
55% 0.786 19.98
60% 0.784 19.92
65% 0.782 19.86
70% 0.780 19.80
75% 0.777 19.74
80% 0.775 19.68
85% 0.773 19.62

Over Wire Measurements for 13/16 - 28 28-UN


* OverWire is a measuring method to obatin the pitch diameter of threads. It is mostly used when a manufacturer does not have a Go/Nogo gauge for a thread. You can learn more about it here

Inch mm
Permissible Wire Diameter Range 0.018 - 0.0361 0.458 - 0.916
Ideal Wire Diameter 0.0206 0.524
Possible Wires
From Standard Sets
* 0.018
* 0.0193
* 0.0206
* 0.0214
* 0.0222
* 0.0241
* 0.0262
* 0.0289
* 0.462
* 0.52
* 0.577
* 0.722
* 0.866

Once you knoe your wire diameter, you need to find out what is the premissable Measurement for your pitch diameter (Depending on the class). Use our Over Wire Calculator to find out!


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