Parker A-Lok 4NU4-B Brass Compression Tube Fitting, Nut, 1/4" Tube OD Nut tightens and provides compression on the ferrule of a flareless fitting Connects to tubing Brass for corrosion resistance, ductility at high temperatures, and low magnetic permeability For use with a twin ferrule compression fitting set for a leak-resistant seal in high-pressure applications For use with copper tubing