The standard: DIN 40431 P1 STEEL CONDUIT THREAD; THREAD GUAGES,
"GO" AND "NOT GO" THREAD RING GUAGES specifies
the requirements for the thread ring gages. A unique aspect of the
Pg Steel Conduit Thread Ring Gage is that the NOGO ring is a plain
cylindrical gage. This actually works because as the thread is machined
the crest of the thread form comes to a point. As the thread size
is changed the position of the point changes and thus can be measured.
The GO ring works in the normal manner by checking the maximum material
condition.