Brass Compression Nut For Plastic, Copper, & Aluminum Tubing, 0.38 Inch Tube OD

That's what this nut is sized for. The 0.375 inch tube OD matches 3/8 inch copper tubing, and copper is one of the materials it's rated to work with. Just make sure you've got the matching sleeve in place before threading it onto the fitting body. Slide the nut on first, then the sleeve, with the threaded end of the nut facing toward the fitting. Once you get it hand tight, give it another 2-1/4 turns with a wrench and you're set.

For 3/8 inch tube outside diameter, this nut matches a 0.375 inch tube OD. It is meant for use with plastic, copper, or aluminum tubing.

It's made to work with aluminum, copper, and plastic tubing. Just be sure to use a 2030 insert for most plastic tubing, except for Eaton PT230.

No flaring is part of the setup here. It is a compression style nut meant to assemble with the matching sleeve on 0.38 inch tube OD, and the basic steps are cut the tube square, slide the nut and sleeve on, bottom the tube on the fitting shoulder, then tighten hand tight and finish with a wrench per the assembly procedure.