The following is to assist those doing this repair who might search the forum in the future. The rocker panel (on a 74' and I assume other C3's) is held on by (6) 10-24 bolts about 1.5 inches long. The first, second and the sixth bolt are retained with nuts accessible from the rear. The problems lie with the middle three which thread into what appears to be a type of GM t-nut insert. The flange of the t-nut appears to be tack welded on the back side to the metal channel that runs under the door. The bolts broke off in all three of these inserts in my situation. Note: There appears to be no practical way to put any heat on the threaded part of the t-nut when it is assembled and I do not think one could get any type of rust penetrant onto the threads so getting out a rusted bolt without bolt failure is almost impossible (in my view).
With the rocker panel removed I initially tried to drill out the inch or so of broken bolt by drilling down the center of the insert. Although it may be possible to do this with an elaborate fixture, I found it impossible to drill out the bolt on the car without destroying the thin walls of the threaded part of the insert. However it was fairly easy to drill out the bolt and the threaded part of the insert together. I found a 10-24 flat flange t-nut insert (ie one without barbs) at the local hardware store that I could install from the back side. It is installed on the top of flange of the original GM t-nut. On reassembly I used anti-sieze on all the bolt threads.
I would note that the repair is actually better than the original design. Should these bolts seize and break in the future (in new t-nut) the broken bolt threads and the t-nut could be easily removed from the rear.
With the rocker panel removed I initially tried to drill out the inch or so of broken bolt by drilling down the center of the insert. Although it may be possible to do this with an elaborate fixture, I found it impossible to drill out the bolt on the car without destroying the thin walls of the threaded part of the insert. However it was fairly easy to drill out the bolt and the threaded part of the insert together. I found a 10-24 flat flange t-nut insert (ie one without barbs) at the local hardware store that I could install from the back side. It is installed on the top of flange of the original GM t-nut. On reassembly I used anti-sieze on all the bolt threads.
I would note that the repair is actually better than the original design. Should these bolts seize and break in the future (in new t-nut) the broken bolt threads and the t-nut could be easily removed from the rear.
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