These metal inserts should be placed into any non-DOT poly-lines that are going to be inserted into either a wrench tightened crush style compression fitting or the newer PTC , push to connect O-ring sealed fittings
It's obvious why you would require a metal insert for "crush" type compression fittings, but dont forget to buy them for your non DOT poly-line to PTC installations either. The insert will keep the poly-line aligned, ridgid, and free from distortion that will occur to a plastic line over time if unsupported
If a DOT compliant installation is required, please use the Fairview 1400 series fittings