This reducing adapter screws into an AN16 male O-ring port and gives an AN12 male 37-degree flare for the hose. It is a common requirement on dry sump tanks, large oil reservoirs and fuel cell plates, which are frequently machined with AN16 bosses while the scavenge, pressure or return lines running from them are AN12. One size step keeps restriction low, and doing it in a single fitting avoids the extra joints and leak paths of stacked reducers. The port seals on its O-ring against the boss face, so fit it without sealant and tighten only until the seal beds evenly.