Threading Data
Formula for Calculation of 60 Degree Threads
 Written by Michael Lerke
From practical shop experience

 

This example uses 8 threads per inch.

The constant for this formula is 1.299 divided by threads per inch (tpi) In this case 8 tpi.

1.299 / 8 tpi = .162375"
This number subtracted
from the O.D. or
major diameter of the desired thread is 100% tap drill size for an internal thread. You would usually want to use between 55-80% so multiply the .162375" by .60 for 60%, for instance and subtract this number from the O.D. of the called thread size.


  

        




To absolute crest
of 60° triangle-.10825"
Now to find the pitch diameter of a given external 60° thread, take 1.299 / tpi (in this case being 8), so 1.299 / 8 = .162375". Divide this number in half to find midway between the top and bottom threads and subtract this number from your nominal O.D. This is your Basic Pitch Diameter.  This is usually the high limit on class 2, 3, and 3a fits. Most class 2a fits are in tolerance with .001"-.002" less.

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