Corrugated Stainless Steel Tubing, or CSST, is a flexible tubing used to supply natural gas and propane in buildings. Compression fittings are used to make transitions between pipes. Here’s how they work:

First, use a steel tubing cutter to cut the pipe to length along a valley in the corrugation.

Then, for three-part fittings like the Home-Flex fitting seen first in the video above, count three valleys in and score the tubing’s jacket with a utility knife. Make another score from that mark to the end, and remove the jacket from the end, exposing two valleys.

Next, slide the nut onto the tubing with the threads facing toward the end being connected. Then slide the retainer ring over the first valley, and pinch it closed using tongue-and-groove pliers (such as Channellocks), applying gentle pressure all around the ring to help maintain its round shape and lock it into the valley.

Finally, insert the fitting and thread the nut by hand. It’s important to spin the nut, not the fitting (typically, the fitting will already be installed on the pipe or tubing it’s joining, so spinning it would loosen that connection). When it’s hand-tight, use a wrench to prevent the fitting from spinning and use another wrench to tighten the nut to complete the connection.

For fittings with multiple rings like this one from Pro-Flex, count 5 valleys in, score the jacket, and remove it to expose four valleys.

Then slide the nut over the tubing with the threads facing the end. Next, open and install the retainer ring in the fourth valley and pinch it closed. Then install the metal washer, followed by the rubber O-ring, which gets snugged up to the metal washer.

Finally, thread the nut onto the fitting—again, only spin the nut—until it’s hand-tight. To complete the connection, use a wrench to prevent the fitting from spinning and use another wrench to tighten the nut a quarter-turn to compress the fitting.

These fittings are easy to install, but this work should be done by a certified installer. Certification training for your particular brand may be available online through the manufacturer.

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