nipple

A nipple is a short length of pipe, typically 6 inches or less, with threads cut on both the inside and outside of each end. It serves as a connector piece between two female-threaded fittings, allowing pipes or fixtures to be joined end-to-end without requiring a fitting body between them. Nipples are manufactured from steel, stainless steel, brass, or plastic, depending on the application and fluid being conveyed.

Nipples are classified by length and material. Close nipples have minimal unthreaded length in the middle, sometimes just 0.25 inches; longer versions are called "street" nipples or standard nipples, typically 0.5 to 6 inches in length. Diameter ranges from 1/8 inch to 2 inches in most plumbing work, though larger sizes exist in industrial applications. The threads are usually male National Pipe Thread (NPT) standard, right-hand, tapered at 1 degree 47 minutes per side. Galvanized steel nipples are common in water lines; black iron in gas lines; brass in higher-pressure or corrosion-sensitive applications.

The primary advantage of a nipple is that it reduces the number of fittings required in tight spaces. Instead of using two elbows and a short length of pipe, a single nipple connecting two 90-degree bends can occupy less horizontal space. However, nipples shorter than about 1 inch become difficult to turn by wrench, and the stub of unthreaded pipe in the center can trap scale or debris in water systems.

Common failure modes and installation practice

Nipples are prone to leaking at either end if the threads are not properly sealed with thread compound or tape. Because both ends must be screwed simultaneously into fixed fittings, over-tightening one end while the other remains loose is a frequent installation error. In old installations, galvanized nipples can corrode from the inside outward, creating pinhole leaks before visible exterior corrosion appears. Replacement nipples in existing lines must match the original diameter and thread pitch precisely.

The term "nipple" originates from its resemblance to a breast or teat, a naming convention found across mechanical trades. In plumbing catalogs and supply houses, nipples are often sold as bulk stock items organized by diameter, length, and material, making them one of the smallest and least expensive connection components in the trade.