I think it is 5/8 if you want to run the tie rod flip bushings but I tought I read somewhere you can just buy the proper ream and just ream the opposite side of the spindle where the tie rod mounts and your done. No bushing. I figured I might be doing this a few times so I might as well buy the ream instead of the flip kit. Maybe I'll see if the kit is reasonably priced then it it probably not worth it.
Jeremy