An AN4 flare union adapter with a male 37° flare at each end, used to join two AN4 female hose ends or ports into one line. AN4 covers a wide spread of small-bore duties — brake and clutch hydraulics, oil pressure and boost gauge feeds, vacuum reference lines and low-volume oil circuits — and the union lets you break a run into sections that are easier to route through a bulkhead or around a subframe, and easier to service afterwards. The 37° flare seals metal to metal on the cone, so thread sealant is unnecessary and can contaminate the system. If the line is hydraulic, bleed properly and pressure-check for weeps before use. Tighten to snug plus a small nip; the flare needs contact, not brute force.