Brass Adapter, 1/8-27 Male NPT To 0.31 Inch Female 42 deg Inverted Flare, 90 deg Elbow

For converting between those two connection types, it’s a 90 degree brass adapter with a 1/8-27 male NPTF thread on one end and a 0.31 inch female 42 degree inverted flare on the other.

Natural gas is actually one of the listed applications, though it's noted as a special order use case. The fitting also covers hydraulic brake, power steering, fuel lines, and transmission cooler lines. The 1/8-27 male NPT thread pairs with a 0.31-inch 42-degree inverted flare on the female end, and that combination handles a solid range of fluid and gas line configurations. The brass construction holds up well across those applications.