An AN4 flare plug that seals with an O-ring into a female AN4 port. AN4 covers small-bore work such as turbo oil feeds, pressure gauge lines and light hydraulic runs, and distribution blocks or tees in these systems often have a spare leg. Plugging it with a purpose-made fitting keeps the assembly serviceable and leak-free. The O-ring seal is more tolerant of repeated fitting than a hard metal cone seal, so this is a good choice for ports you expect to reopen during development. Nip it up only far enough to compress the seal. Verify the O-ring suits the fluid, particularly on hot oil circuits.