Hello irena2053, I am going to go ahead and take a stab at your
question. Here is a very informative website concerning "J series"
The UNJ for internal threads is identical to the UN thread except that
the UNJ minor diameter is slightly larger than the UN minor diameter.
This is to allow the minor diameter of the internal thread to clear
large root radius of the UNJ external thread. If a UNJ bolt is screwed
into a UN nut there will in most cases be an interference fit at the
thread minor diameter.
The rounded root of a UNJ external thread reduces the rate of
threading tool crest wear and greatly improves fatigue strength over
that of a flat root UN thread.
External UNJ threads per SAE AS8879 have a rounded controlled root
radius at the minor diameter. The minimum root radius is calculated at
0.15011p. The root contour shall have a smooth, continuous,
nonreversing contour and shall blend tangentially into the flanks and
any straight segment. The radius is larger than the UNR style.
Because of the large root radius on the minor diameter of the external
thread, the UNJ GO ring gage is supplied with an enlarged minor
designed to clear the root radius. The GO ring gage is the only gage
that is modified specifically for the UNJ specification and the only
difference between the UNJ GO and the UN GO thread gages is the minor
diameter. The UN GO ring gage if used to measure the UNJ thread the
gage will reject the part because of the minor diameter interference.
The UNJ NOGO ring gage is identical to the UN NOGO ring gage except
that it may be marked with a "J". The minor diameter of the NOGO ring
gage is relieved to just below the pitch diameter, and because of this
the standard UN NOGO gage will clear the minor diameter of both the UN
and UNJ external threads.
The set plug members used for the UNJ series threads are identical to
the set plugs used for the UN series. If UNJ gages are ordered from a
gage maker, the gage maker will supply the standard UN gage members
with a handle marked UNJ. This is because set plug gages do not check
the minor diameter of the ring gage they are setting. The minor
diameter of the ring gage should be checked with plain pin gages after
the ring has been properlly set with the set plug.
If you are not familiar with basic UN thread details, here is a good drawing:
So, after all this information, I think the answer to your question is:
"If a UNJ bolt is screwed into a UN nut there will in most cases be an
interference fit at the thread minor diameter."
In other words, you can't screw a UNJ bolt into a UN nut because the
flat crest of the UN thread will interfere with the large radius root
of the UNJ thread.
However, "The UNJ NOGO ring gage is identical to the UN NOGO ring gage
except that it may be marked with a "J". The minor diameter of the
NOGO ring gage is relieved to just below the pitch diameter, and
because of this the standard UN NOGO gage will clear the minor
diameter of both the UN and UNJ external threads."
You did not say in your question whether you were talking about a GO
or NOGO gage. If you have further questions, please ask for a