A field replaceable wing nut having an arcuate body and an arcuate insert is disclosed. The wing nut is designed to replace an existing wing nut which has deformed or non-useable lugs on a hammer union connection. The wing nut has accurate alignment of the mounting threads using an alignment attachment device. Replacing the wing nut in the field does not require any special equipment or training. Each wing nut is designed for a particular pipe diameter.