Accurate Formula to Compute the Pitch and Major / Minor
Diameters for Before Plate Thread Plugs and Thread Rings**

Courtesy Pennoyer Dodge

If no plating tolerance is given, then it is assumed that the maximum plating thickness will be nominal or minimum thickness given plus 50% (see ASME B1.1-1989, pg. 16, 7.4.2; pg. 18, 7.5.2).  The diameters of the gaging for external threads will be smaller while the diameters of the gaging for internal threads will be larger, than standard.

Example
Based on .0002 to .0003 allowance per side (multiply by 4)
.0002 X 4 = .0008 MIN
.0003 X 4 = .0012 MAX

1/4 - 28 UNF 2B B / P
Basic GO P.D. .2268 + .0012 MAX = .2280 GO P.D.
Basic NOT GO P.D.  .2311 + .0008 MIN = .2319 NOT GO P.D.

Basic GO major .2500 + (.0012 / 2) = .2506 B / P GO major.
Basic NOT GO major .2466 + (.0008 / 2) = .2470 B / P NOT GO major.

Thread Rings

For the GO ring, subtract the MAX amount of plate.
For the NOT GO ring subtract the MIN amount of plate.
For the minors of thread rings, decrease the minor diameter on the GO thread ring by using half the MAX amount of plate.
On the NOT GO thread rings, decrease the minor diameter by using half the MIN amount of plate.

Example
Based on .0002 to .0003 allowance per side (multiply by 4)
.0002 X 4 = .0008 MIN
.0003 X 4 = .0012 MAX

1/4 - 28 UNF 2A B / P
Basic GO P.D.  .2258 - .0012 MAX = .2246 GO P.D.
Basic NOT GO P.D.  .2225 - .0008 MIN = .2217 NOT GO P.D.

Basic GO minor .2103 - (.0012 / 2) = .2097 B / P NOT GO minor.
Basic NOT GO minor .2148 - (.0008 / 2) = .2144 B / P NOT GO minor.