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What Is a Metric Flange Nut?

By B. Leslie Baird
Updated May 17, 2024
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Metric flange nuts are designed to fit on metric-sized bolts or threaded rods. The large circular base on a flange nut provides even distribution of pressure and creates a firm hold. More torque is required to loosen the nut. Metric measurement systems are common throughout much of the world, while the US uses a mix of metric and standard, or English, measurements.

The flange portion of the nut may have slight grooves or serrations in the bottom that are designed to provide additional strength to the locking power of the fastener. A metric flange nut is more resistant to loosening itself due to vibration, shock, and thermal stress. This makes them desirable for use in aviation and other high demand applications.

Hexagon-shaped heads are common on a metric flange nut. This shape allows for a solid gripping surface for sockets and open or box-end wrenches. Square heads may also be used, but they do not offer as much gripping power for tools. Flange bolts will also normally have a hex head.

Swivel and spherical flange nuts offer additional benefits for specific applications. The swivel variety, for example, gives the same locking power as a serrated base, without damaging the surface it contacts. Flange bolts have the same feature of the wider contact surface beneath the head of the bolt. These bolts may also have serrations on the contact surface.

A metric flange nut is identified by letter and number, the most popular being M3 through M20. The letter M simply identifies the part as being metric. An M5, for instance, has an exterior diameter of 0.46 inch (11.8 mm), and a height — or thickness — 0.20 inch (5 mm). Flat to flat distance of the hex is 0.315 inch (8 mm), and point to point is 0.346 inch (8.79 mm). The number 5 designation is the interior thread diameter of 5 mm, or 0.20 inch.

Various materials can be used to manufacture a metric flange nut. The most prevalent construction is hardened steel with a zinc coating. Grading on steel is an indicator of the alloys in it, mechanical properties, and corrosion resistance. Some of the common grades of steel include 316 stainless, 18-8, and A2.

Bolts and nuts may also include grade markings for strength. A grade 5 equals 120,000 pounds per square inch (psi), indicating the amount of durability and strength. Metric flange nuts are often graded as 8.8, 10.9, or 12.9. Markings on metric nuts are normally found on the top of the nut for easy identification of the strength.

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