UNJ internal threads simply have a slightly larger minor diameter to allow for the larger root radius of the UNJ external (male) thread. For a cutting tap the pilot hole size isn't critical and you can probably use a normal tap drill unless you're dealing with hardened steel. If you're using a roll tap then you should be more careful with it. Minor diameter is easy to find . . . just look in Machinery's Handbook.
Mark Stapleton
Watermark Design, LLC
Charlotte, NC
A cutting tap does not cut/form the minor diameter, just the flank angles and major diameter. The pre-tap hole diameter is not a "pilot" hole, it is critical and must meet the modified minor diameter requirements of the UNJ thread prior to tapping.
Don't think I've ever seen roll taps successfully used for UNJ threads. Acceptable roll tap life generally requires an increase in finished minor diameter size (less % full thread) than permitted by UNJ thread minor diameter limits. Class 3B thread tolerance requirements and typical "aerospace" materials generally aren't "roll tap friendly" either.
