I am not a ME, before going Digital in ’67, I was civil. My dad was an A&E Licensee after #2 who retired from an Aircraft Manufacturer. My early learning curve was punctuated with the fact that his Reed & Prince had a tan handle and the Phillips did not. 
My question about bolt length for the Backbone was because my dad always said ‘that all fasteners have specifications’ (inees and outees). His legacy about this has provided me with drawers of fasteners that he sorted from the 5 gal buckets of sweepings that the surplus store sold. The cardinal rule of the assembly line being ‘if you drop something, do not pick it up, it could be the wrong fastener’.
Steve
Steve
I too am an A&"
P" there are a lot of different aircraft bolts and each seems to have its own set of rules for how they are used. The most common aircraft bolts are "AN" and the general rule on bolt length is that the shoulder or unthreaded part should be at least as long as the depth of the hole you will be placing the bolt into. if the bolt is longer then you use a washer to shim the distance of the bolt so that the nut won't bottom out.
However what I suspect that GM did with the bolts for the brace is that they technically are not bolts but cap screws. Meaning that there is no shoulder to the bolt but that it is threaded to the head. Anyone care to correct me on this one?
You are also right to wonder how much thread engagement is taking place when you substitute a thicker plate of aluminum in for the original brace. Unfortunately I can't answer that but if you were to replace the bolts with ones that were longer by the amount of thickness of your new brace you couldn't go wrong.
As far as hole diameter if you stay with the same size hole that is in the original brace you should have no issue. The proper torque was also listed on this thread so as long as your new bolts are the same grade and thread as the originals you should use the same torque.
I am guessing that the GM bolts are metric in which case you will have a problem trying to figure out which of those surplus AN bolt to use as they are all standard. Its really kind of odd when you work on an American car and have to use a metric wrench but a German, Swiss, or for that mater most all other country aircraft you use standard wrenches (except for those F'n French made planes, there metric, and those British warbirds, they used BS or British Standard). P-51 Mustangs are another story as they used an American made version of a Rolls Royce engine, standard and BS.
