This Watts union is commonly used in gasoline, grease, oil, water, vacuum and air line applications to join pipes and tubes together. It is made of brass for strength, durability and rust resistance. The reducer union fitting features a 3/8" outside diameter compression tube end and a 1/4" outside diameter compression tube end. It is ideal for use with copper, brass, aluminum or plastic tubing. The pipe fitting is not recommended for use with steel tubes.