When replacing your blue 5 mm. or 6 mm. ARB air line with steel line with -3AN ends, you need an adapter. 

The 1/8" NPT adapter is what you need at your regulator outlet port, your pneumatic toggle switches, or in a standard air manifold with pipe thread ports.

The 3/8" x 24 adapter screws into the brass axle housing bulkhead fitting. 

The 1/8" BSP adapter screws into your solenoid outlet port.