I am installing an Optima Yellow Top behind the driver's seat and want to ground the negative side to the chassis.  There is a threaded insert on the right side, a couple of inches in front of the post that supports the lid.  I'd like to strip off the paint and use it, but I can't tell what the thread size is.  Anybody know what the size is?

Or... what's a good option for making the ground to the chassis?

Tom Salicos
'87 Syncro Westy EG-33