17,23,34 are inches, 42,57,86 are millimeters,
17*2.54=43.18……
According to American standards, NEMA17 is equivalent to 42 motors, 23 is equivalent to 57 motors, and 34 is equivalent to 86 motors.
42, 57 and 86 refer to the external dimensions of the motor flange surface.
Related link : Nema 34 Closed Loop Motor Kit