Flare Fitting

Flare fittings work by deforming the end of tubing outward into a 45-degree cone that seats against a matching conical surface inside the fitting body. The nut compresses the flared end against that seat to create a watertight seal. The tubing itself gets flared using a flaring tool before assembly, so you're creating the seal geometry on the tube end, not the fitting.

These fittings are common on fuel lines, gas appliance connections, and soft copper refrigerant lines. They're also standard on old water-supply connections where soldering wasn't practical or code-compliant. The big advantage is you don't need heat or solder—critical when working near combustibles or in spaces where an open flame is unsafe. Flare fittings are reusable if you're careful; you can unbolt them and re-use the same tubing by re-flaring it.

The catch: flare fittings need proper technique to avoid leaks. Tubing has to be dead-straight and properly flared—too shallow and you'll weep, too deep and the tube splits. They also won't work with hard copper tube or plastic. Most plumbers encounter these on gas work or older house connections, and they're worth knowing because a lot of old stock still uses them and they're faster than soldering in tight quarters.