Brass Compression Fitting For Plastic, Copper, & Aluminum Tubing, 0.50 Inch Tube OD To 1/2-14 Male NPT, Straight

Not all plastic tubing works the same way with compression fittings. Some plastics are soft enough that the ferrule can deform the tube wall during tightening, which leads to a weak seal over time. For those cases, an internal support insert is needed to keep the tube from collapsing. The product does accommodate plastic tubing, but whether your specific plastic type requires an insert depends on the material. Copper and aluminum tubing generally don't have that issue and can be used without inserts.