LASCO 17-6783 1/4-Inch Female Flare by 1/4-Inch Male Pipe Thread Brass Adapter
This fitting could be used in 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.
Update (slightly lower price) 4/18/18 7 PM Central: This has dropped to $8.17. That’s a 18% discount from list and the best historical direct from Amazon price I found. Shipping to most US addresses is also free with Prime Membership [Try Prime for Free] or a qualifying order. Price, shipping and availability can change quickly. Check the product page for current price, description and availability. Also: Recent Amazon Finds and Amazon Fillers Resource Page
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