AN3 is the smallest size in common use, found on brake and clutch lines, oil pressure gauge feeds and other low-flow signal plumbing. This cap threads onto a male AN3 flare fitting and seals on the 37-degree cone, which makes it genuinely useful for keeping fluid in and air out when you disconnect a gauge line or leave a caliper port open during a rebuild. It is also a tidy way to blank an unused port on a master cylinder adapter or distribution block. Being a female cap it fits male AN3 threads only — a female AN3 port needs a plug rather than a cap.