Adapt a Plastic Water Line Without Leaks: Plastic Ferrule Plus a Support Sleeve

The Problem

A compression fitting made for copper gets used on plastic tubing with the same brass ferrule, and it seems fine — until the plastic creeps, the joint loosens, and the tube blows out of the fitting with the water on.

The Technique

Adapting a plastic line to a metal fitting takes three parts, each in the right material, stacked in this order:

1. The metal nut goes on first — metal threads on metal fitting, always.
2. A plastic ferrule, not a brass one. Plastic against plastic lets the ferrule compress evenly around the tubing without cutting it.
3. A support sleeve (stiffener) inside the tubing — this is the part that gets skipped. It backs the tubing wall up from the inside, so when the ferrule squeezes down, the tube can't collapse.

Without the sleeve, tightening the nut squeezes plastic onto plastic with nothing behind it — the tubing deforms, and the whole line can pop out of the fitting. The creator makes the point by assembling one without the sleeve and pulling the tube free by hand.