"Ned Simmons" wrote in message
...
On Sun, 14 Jul 2013 22:06:34 -0400, "Jim Wilkins"
wrote:

This one is easy to check. I measured a few 1/4" and 3/8" Greenfield
taps at 0.0015 - 0.002" under the nominal diameter.
jsw


Then they won't cut proper UN threads. The *minimum* major diameter
for 1/4-20 UNC - 2B (and 1B & 3B) internal threads is .2500.
Likewise,
the minimum major diameter is equal to the nominal size (the basic
major diameter) for all 1B, 2B and 3B UN threads. Similar rules
apply
to metric threads.
Ned Simmons

OK, many of my tools were second-hand. New 1/4, 5/16 and 3/8 taps
measured 3-4 thousandths over nominal size.
jsw