10mm x 1.5 thread. If you are talking about the very top bolt that goes through the alternator and metal bracket and the tubular brace that goes to the back of the alt and the exhaust header, then it is aprox. 2.25" long. (going off of memory 'cause I'm not looking at one directly) You can use a 3/8-16 bolt and nut if that's all you can find. Maybe someone on here has a sprae they can part with, too.