Pn# - UN04 - 5pc Solid Brass 5/8-18 Female Inverted Flare Union For 3/8" Fuel Lines. Unions are used to join two brake, fuel, or transmission lines together. This may be needed for the long brake line that runs for the front of the car to the rear axle. Use for joining 3/8" Domestic Fuel lines. Match the thread and flare type to the lines being connected. is important that both flare type and thread match for a leak-proof connection. SAE 3/8 Tube size x 5/8-18 thread size