Thanks for response. I did some early research and saw that as well, but it is not. I believe that may be for the studs on the negative cable clamp. I've tried M6x1.0, M7×1.0, and 1/4-20. Unfortunately this nut and bolt don't want to separate fully without some excessive force and not sure if I'll damage thread if I remove the nut fully so haven't been able to take into HW store on a thread gauge. Will try 1/4-28.