Imperial Flare Nut Wrenches Secure Tools supplies Imperial Flare Nut Wrenches. They are similar to metric flare nut wrenches but designed for use with imperial (inch-based) measurements. These wrenches are specifically used for working with flare nuts and fittings in applications that follow the imperial measurement system. Imperial flare nut wrenches feature a closed-end opening with a small opening on one or both sides. The closed-end provides a firm grip on the flare nut, while the small opening allows the wrench to be slid over the tubing or fitting without fully disconnecting it.