Metric Flange Nuts

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Metric Flange Nuts are basically a hex nut with an enlarged circular base flaring out from the bottom of the nut, and a metric thread pitch. 

The bearing surface on standard metric flange nuts is smooth.

Locking Metric Flange Nuts have serrations that grip the bearing surface so that significantly more torque is needed to loosen than to tighten the nut. These metric flange nuts are designed for use in applications where loosening forces are present, such as vibration.