just throw any old grade 8 bolt/nut/washers in there with a similar diameter. as long as your not wheeling nothing will get messed up if the bolt diameter has some slop.
Nuts are cheap. *snicker*
Just stop by Lowe's or HD and grab one 16mm fine and one coarse thread. And one 14mm fine and one coarse thread. Screw one on to see what fits, return the other three. If you have calipers, measure the outside dia. It will cut the guessing game down.