Eric,
I brought the #6 ground wire to one of the tabs on the back of the H.D. starter. That (body of the starter) is where the heavy current flows, and that is also directly connected to the engine of course. That is where they connect the engine ground on the S-19 Rans. I brought a much smaller wire/jumper (#14) from there up to the hole on the cylinder just for the heck of it, so the important ground for the ignition system has 2 connection paths at the engine. That threaded hole is indeed 6 mm. I got a bolt at my local Ace Hardware.