Hi, welcome to Just Answer.
Below are the procedures.
Open cover -1- for battery case and unhook.
CAUTION: Never disconnect or connect the positive B+ terminal from battery while the negative GND terminal is connected Danger of short circuit!
Yes, it's a 8mm x 1.25 thread pitch...but if you can't find the original bolt I would just get a new one from the dealer...it fits the best in there and is about $3.