This adapter threads into an AN16 male O-ring port and presents an AN20 male 37-degree flare for the hose. It is a bulk-flow interface fitting, used where an AN20 line runs to a tank, reservoir or housing that is only machined for AN16, typically on breather, coolant transfer and large oil reservoir plumbing. The AN16 port remains the restriction, so the reason to use it is hose continuity and routing rather than added capacity. Sealing is by the O-ring against the machined boss face, so it is installed dry; make sure the face is clean and the fitting seats squarely before tightening.