An aluminium weld-in bung producing a male AN12 flare port. AN12 is a large, low-restriction size reserved for the jobs that really move fluid or air: dry sump scavenge lines, oil cooler plumbing on high-output engines, crankcase breather and vent runs, and large surge tank feeds. Aluminium means TIG welding into alloy fabrications such as fabricated sumps, billet or sheet surge tanks, catch cans and header tanks. A port this size needs a careful welding sequence, so tack at opposing points and let it cool between passes to avoid pulling the flange out of shape. Seal is on the 37-degree flare, not the thread.