Anti-vibration thread form

A vibration resistant fastener assembly comprising a nut body having an aperture provided with a helical internal thread, and a member having a substantially V-shaped symmetrical external helical thread with a standard leading and following flank angle of sixty degrees relative to the central axis of said member and adapted to mate with said helical internal thread, the internal thread in said nut body having a thread form comprising a leading flank and a following flank, said following flank having a concave surface such that all tangents thereto extend at an angle less than sixty degrees relative to the central axis thereof and less than the flank angle of the following flank of the external thread on said member whereby the radially outermost portion of said external thread is initially engageable with the concave flank of said internal thread.