A nut and bolt securing device is disclosed to prevent loosening of the assembly due to environmental conditions. In a first version of the invention the two piece device is fitted over a nut and a portion of a bolt extending from the nut, the two sections of the device being secured together with set screws, with serrations within the device, and one or more set screws engaging the front end of the device. In a second version of the device a shelf like planar projection on the front end of the device engages a planar surface on a nut, again combining with the serrations within the device to prevent fastener loosening. The device is particularly described in use in conjunction with refrigerant system flare nuts so as to prevent loss of refrigerant at these flare nut unions.