A possibly useful observation, I haven't worked installation on rigid in over 40 years and may well be out of date. This thought came up suddenly, out of the "clear blue", as it were.
Conduit thread and pipe thread are functionally the same, except:
Pipe thread has a taper where conduit thread does not. The NEC Code
allows pipe thread to be used for conduit but it's
not a preferred method. Conduit couplings have a straight thread with a very slight taper, not really enough to notice. As does "store bought" conduit. The difference can only be seen laying a stick of conduit alongside a stick of pipe the same size.
There are several combinations involved here that may have a bearing on the situation. Not liking Chinese made tooling, they are my first thought. That being that when they first started making threading dies, they measured a piece of conduit. Thereby making a conduit thread through lack of knowledge of what they were copying. No further comment, it gets political. . . But a stick of conduit is 10 ft long while pipe is 21 ft. Much easier to handle. . .
The next possibility is "store bought" couplings having conduit thread, being placed for sale as plumbing fittings through the error of, again, not knowing what they were doing. Which is a fairly common problem in today's world. Most conduit devices are specific to electrical work and most pipe fittings to plumbing. They would not normally even be in the same building but for Lowe's and Home Depot and the like where the help has trouble reading labels. More politics. . .
There are several other posiblities where electrical threads and plumbing threads have gotten mixed. This comment is just what sprung to mind first. It may well be you have acquired, mistakenly, conduit threading equipment.
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