This adapter threads into an AN12 male O-ring port and presents an AN16 male 37-degree flare to the hose. It is used where a large AN16 line, typically a dry sump tank feed, breather or coolant run, has to terminate at a component only machined for AN12. The AN12 port remains the narrowest point in the joint, so the purpose here is to keep a single large hose size through the run rather than to increase capacity. The O-ring seals against the machined boss face, so fit it dry, seat it squarely and avoid over-torquing it into an alloy casting.