1/2" x 13 stainless or grade 8 steel... The hardness of the grade 8 isn't necessary, but they stand up against corrosion a little better than the standard grade 5... If you go stainles, don't forget to go back a few times over the course of a month or so and check the torque and be sure to use anti-seize when you install... If you re-torque, be sure to spin the nut only and not the bolt to keep your seal... They'll stretch a little with the initial torque, but they'll eventually find a happy spot... The grade 8's won't need torqing but once, and 45-50 ft. lbs. of torque should be plenty on either fastener... You might have known all of this already, so I'll shut up..