I did this on my Hurricane. I used a fairly long M6 bolt and a 15/64" drill bit. You want to make sure the bolt is long enough that it has an unthreaded portion. I went to the effort of drilling the factory plastic steering knuckle, tapping it, then through-drilling it with the 15/64" bit far enough that the bolt could thread in. I ended up installing a nut (with loctite) when all was said and done though. You don't want it tight since the steering bar needs to be able to swivel relative to the bolt, and the loctite prevented the bolt from threading out of the plastic assumbly.
I had to remove some of the Hurricane's plastic to clear the higher-profile bolt head, and did some incremental reaming of the hole through the steering bar until I was happy with the fit (6mm bolt is slightly larger than a 15/64" drill bit; the plastic expands but the metal doesn't
F-150 - 24-36V with homebrew 100A variable-speed motor controller, 4x 775 motors (no, it's NOT all-wheel-drive--look HERE
for more details!)
Pink Rocket Princess Mustang - 15mph, 36V, 4x 775 motors, homebrew PWM controller rev 2....
Hurricane - 24V w/50A step-down controller, 775 motors