okay - yes, so the one purchased from Amazon was definitely not 5/16 despite it being labeled as such. I bought a new one that actually is 5/16 and it works great! Thank you all for responding.
Hello - does anyone know the size truss rod wrench needed for a J-185 Acoustic reissue? I purchased in the early 2000s. I bought a 5/16th and it's too small and wanting to confirm prior to buying another size. Thank you for the help!