You dont need a spec. You only have to figure out how long the bolt should be. As a general rule the bolt should go all the way through all supported members and thread fully into the nut. In other words the bolt end should go into the nut until it is at least flush with, but preferably protruding through the nut by a few threads. To determine the length of bolt you need, add together the thickness of the additional components and add that number to the length of the stock bolt. Get the bolts from your local bolt supplier. A place like Fastenal or Granger . I get mine from Conroe bolt just down the road from my shop.
