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2A and 3A Ring Gages


QUESTION: I have a 6-32 ring gauge for both 2A and 3A class of thread.
Why do 6-32 screws feel tighter in the 3A Go gauge than the 2A?
A 3A thread's maximum Major and Pitch diameters are larger than that of a 2A thread and I would think that the 3A Go gauge would therefore be larger.
All gauges have been calibrated.


See the Table below, showing the comparison of the Pitch Diameters of the #6-32 UNC Threads.
Please note that the 2A External Thread has an allowance built in to the Maximum limit (.0008).
This .0008 is below the Basic Pitch diameter to allow for clearance at the opposite extreme limits.
The 3A has the Max Pitch Diameter the same as the 2B Minimum.
There is no allowance for fit at the opposite extreme limits.
If you have an External 3A thread that is at the Maximum limit (PD .1177) & the Internal 2B Thread is at or close to the minimum limit (PD .1177) you will have a very tight thread or one that may not assemble.
This is generally not the case as most product threads do not run at the extremes.

Mike Oddy

#6 - 32 UNC Threads

Pitch Diameter

Pitch Diameter
Class of Fit Min Max
Class of Fit Min Max
2B 0.12 0.12
2B 0.12 0.12
2A 0.11 0.12
3A 0.12 0.12

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