Frames have to flex. Therefor they are riveted. The theory behind rivets is that they fill up the hole completely instead of relying on the squeeze of the cap and nut. We use a ribbed carriage bolt to replace rivets when we attach trailer cross members to the side rail. The ribbed shank fills up the hole like a rivet. http://www.imperialinc.com/grp040.shtml The same company also sells a flange bolt which we use in truck frames. We use 5/8 which of course be over kill for a pickup.