John Guest FNPT x Tube Conn. 3/8 x 3/8

The John Guest Female NPT x Tube Connector, 3/8" x 3/8", from Reliance Worldwide Corporation links a 3/8" female NPT threaded port to 3/8" push-fit tubing. It is a frequently used fitting for connecting flexible water lines to threaded components in filtration and reverse osmosis systems, bridging rigid hardware and flexible tubing in a single, compact piece.

One end accepts a male NPT thread, letting you screw the fitting onto a threaded outlet, valve, or component. The opposite end is a John Guest push-in collet that grips 3/8" tubing and seals it with an internal O-ring the moment the tube is inserted. This combination removes the need for stacked adapters and keeps the junction low-profile, which is welcome in crowded under-sink and equipment cabinets.

Where It Fits

• Connecting tubing to threaded filter housings and manifolds
• Adapting valves and components to push-fit tube runs
• Point-of-use and point-of-entry water treatment plumbing
• General plumbing repair and remodel work
• Linking pumps, gauges, and accessories to a tube circuit

To install, wrap the mating male thread with PTFE tape, thread the fitting on hand-tight plus a slight turn, then cut your 3/8" tube square and free of burrs and push it fully into the collet until it bottoms out. A gentle tug confirms the grip, and the O-ring seals the joint automatically. The push-fit side is fully demountable, so you can release the tube for servicing by depressing the collet, then reconnect when the work is done. Because the body resists corrosion and scale, it holds up to continuous contact with treated water far better than many metal alternatives. With its straightforward thread-to-tube transition and tool-free tube connection, this John Guest female NPT connector is a reliable building block for water treatment installations large and small.

Keeping a few of these connectors on hand covers one of the most frequent needs in any treatment build, since threaded female ports appear on pumps, gauges, valves, and housings throughout a typical system. Paired with the rest of the John Guest range, this fitting lets you adapt nearly any threaded component to push-fit tubing, building a complete layout from parts that all assemble and release the same way.