Guest 3/8x1/4 R Union

The John Guest 3/8" x 1/4" Reducing Union from Reliance Worldwide Corporation joins two different tube sizes inline, letting a 3/8" line transition cleanly to a 1/4" line without threads or tools. It is a common fitting in water filtration and reverse osmosis systems, where supply and distribution tubing frequently differ in diameter and need a tidy, leak-free connection between them.

Mixed tubing sizes are routine in point-of-use treatment plumbing. The larger 3/8" line often carries the main supply for better flow, while 1/4" tubing feeds smaller components such as RO membranes, post-filters, or a drinking-water faucet. This reducing union bridges the two with a push-fit collet on each end, eliminating the need for stacked adapters and keeping the junction compact.

A Clean Size Transition

• Connection: Tube x Tube, 3/8" to 1/4"
• Straight reducing union for inline size changes
• Corrosion-resistant polypropylene construction
• Push-in assembly grips and seals each tube automatically
• Demountable for easy line servicing and reconfiguration
• Reusable across multiple installs and repairs

Installation is quick. Cut both tubes square and free of burrs, then push the 3/8" tube into the larger port and the 1/4" tube into the smaller one, seating each fully past the collet. A gentle tug on each side confirms the grip, and the internal O-rings create a watertight seal at both ends automatically. Because polypropylene resists corrosion and the chemicals in treated water, the union holds up to continuous service without scaling or rusting. To disassemble for maintenance, depress the relevant collet and withdraw the tube undamaged, then reconnect when the work is finished. For reliable, reusable size transitions in residential water treatment, this John Guest reducing union is a dependable solution that keeps mixed-diameter runs neat and secure.

Keeping a few reducing unions on hand makes mixed-size layouts far easier to plan, since you can run a larger supply line where flow matters and step down only where a component requires it. Combined with the rest of the John Guest range, this fitting helps you build a clean, interchangeable system that adapts as needs change, all without soldering, crimping, or specialized tools at any junction.