An AN12 flare plug that seals on an O-ring into a female AN12 port. At this size you are typically working with suction-side fuel plumbing, surge and swirl tanks, dry sump scavenge lines and large breather circuits, where components often carry more outlets than any single build uses. Blanking a spare port properly is important on the suction side in particular, since a poor seal there draws air rather than leaking fluid and can be difficult to trace. The O-ring seal allows removal and refitting without damaging the flare seat. Tighten only enough to compress the seal, and check O-ring compatibility with the fluid and temperature involved.