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Cannot find the correct hole size for a 1.5"x 8 tap

I'm beginning to think it ought to be another requirement to read a list of things before posting any questions. :-) that "tap drill" for nominal 70 pct thread is "nominal diameter" minus "pitch" no matter what system or where you are. :-)
..lewie...
 
Let's clarify Lewie's comment a bit . . .the hole-to-be-threaded diameter can be calculated as "Major Diameter minus Pitch" for three commonly-used threadforms -- ISO Metric, Unified (aka ISO Inch), and Sellers (aka Franklin Institute, US Standard, and American National) -- which all have a 60 degree flank-to-flank angle and a 1/8 Pitch flat at the Major Diameter.

This calculation works with any combination of Major Diameter and Pitch, whether standard or special (aka oddball) of ISO Metric, Unified, and Sellers threadforms, but doesn't work for the UNJ, MJ, or most other threadforms.
 
Let's clarify Lewie's comment a bit . . .the hole-to-be-threaded diameter can be calculated as "Major Diameter minus Pitch" for three commonly-used threadforms -- ISO Metric, Unified (aka ISO Inch), and Sellers (aka Franklin Institute, US Standard, and American National) -- which all have a 60 degree flank-to-flank angle and a 1/8 Pitch flat at the Major Diameter.

This calculation works with any combination of Major Diameter and Pitch, whether standard or special (aka oddball) of ISO Metric, Unified, and Sellers threadforms, but doesn't work for the UNJ, MJ, or most other threadforms.


IME - though I haven't checked every pitch to the 4th decimal place, it works for Whitworth standard threads (55deg angle) as well, ............... as it has done since Sami started drilling tapping holes a long long time ago. ;)
 
When in a pickle on an oddball. Besides all the other good suggestions here.

I would look for a similar diameter thread with the same pitch. 1-8" is common.
Just add a half inch to everything. That'll even be close enough to make actual
go-nogo gages.

But.. Diameter minus pitch will always get you where you need to go, as long as
nobody is inspecting it.
 








 
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