Hi everyone , this concerns a mexican 5 string bass neck . The truss nut was a 3/16 hex (I think) and well inside the hole . Somehow the hex hole has sheared open so nothing will turn it. I removed the fretboard from nut to 1st fret , the only way I could get the nut off was to drill a hole through it and turn it , then drill another hole and turn it . I got it off and ordered a new nut from Fender , but the new nut is 3/16 but doesn't fit the rod ! The rod is closer to 5mm but won't take a 5mm nut . I hope someone has more experience than me in this dept. The serial number is MZ8073516 , and the butt has MO1784887. Thanks .
I now know that on 5 strings the truss nut is 5/16" and have ordered one .