...They had extremely fine threads to screw on the unthreaded conduit ....Set screw / threadless fittings will not work...
What kind of pipe are you talking about?
342.28 ~ IMC
344.28 ~ RMC
Where conduit is threaded in the field, a standard cutting die with a 1 in 16 taper (3/4-in. taper per foot) shall be used.
Don't get mixed up with pipe size/type and your threadless or compression fittings. For example, a 3/4" RMC will NOT accept a 3/4" EMT compression fitting....likewise, a 3/4" EMT will NOT accept a 3/4" threadless fitting.
Threadless and compression are two different animals.