Pack of 10 Eaton Weatherhead C5205X4X4 Carbon Steel 1/4″ NPT Male x 1/4″ JIC Male Fittings
These fittings could be used for a lot of things including putting together a Spunding Valve Build. A Spunding Valve allows you to ferment under pressure, naturally and precisely carbonate in the keg, fix over-carbonated beers and more.
See: [Build a Spunding Valve]. Take a look at Build Option 2 for a version that uses a similar although not identical fitting.
Eaton Weatherhead C5205X4X4 Carbon Steel SAE 37 Degree (JIC) Flare-Twin Fitting, Adapter, 1/4″ NPT Male x 1/4″ JIC Male (Pack of 10) – note that multiple variations of this product may be available, as such a different version may appear at this link
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Make sure the components you use are compatible and rated for your intended application. Contact manufacturer with questions about suitability or a specific application. Always read and follow manufacturer directions.