Which of the following fitting types does NOT require O-rings?

Flared fittings do not require O-rings because they rely on a tapered connection that creates a tight seal when two metal surfaces are pressed together. The design of a flared fitting involves a cone-shaped section on the end of the tubing that flares outward, allowing it to seat firmly against a similarly tapered receiving component. This mechanical seal does not depend on the compressibility of a material like rubber in the form of an O-ring, making it effective in high-pressure applications where reliability and strength are critical.

In contrast, bossed fittings, flat-faced fittings, and flanged fittings typically use O-rings to ensure a leak-proof seal by allowing for some flexibility and compensating for any irregularities in the mating surfaces. These fittings rely on O-rings to provide the necessary compression and sealing ability, particularly in dynamic applications or where there may be slight movements between the connected parts.