Abstract: A nut assembly to be mechanically locked to a threaded bolt or pin of the type having an annular groove. The nut assembly comprises a threaded nut element to engage the bolt and a washer connected to the nut element. The washer has first and second coaxially aligned cylindrical walls extending therefrom. The first wall of the washer is deflected into contact with the nut element to prevent the disconnection of the nut element from the washer during handling prior to installation. In the as-packaged configuration of the nut assembly, the nut element is seated upon the second wall of and spaced from the washer. During installation, the nut element is drawn towards the washer to deflect the second wall radially into the annular groove of the bolt to mechanically lock the washer to the bolt and thereby prevent the inadvertent withdrawal of the bolt from the nut assembly due to vibration and similar mechanical forces.