There are straight pipe threads, think electrical rigid conduit.
NPSM – free-fitting mechanical joints Thread
NPSM is a lesser-known pipe thread standard series defined in
ASTM B1.20. This thread series creates a non-pressure tight connection. It is mainly used in mechanical joints and special applications with no internal pressure and, consequently, no sealing requirements. It is possible to achieve a seal with the aid of an O-ring.