Chase nipple is 3-1/2 inch and are malleable iron. Chase nipples are used through a knockout to connect a box to a conduit coupling, with a locknut to connector two boxes side by side or to connect fixture housing to a continuous run.
Chase nipples can be used:
Through knockout to connect box to conduit coupling
With a locknut to connect two boxes side by side or back to back
With a locknut to connect fixture housing to continuous runs