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Please be more specific about the bolt you are referring to.
The bolt is a 7/16” diameter hex head. The bolt goes through an aluminum body mount. It is located behind the right rear wheel. It is the furthest rear body mount bolt on the right side of a 72 C3.
Are going to take the body off the frame? Coupe or convertible? That bolt goes into the interior far back corner in a recessed pocket then into a cage nut that is riveted to the body.
Odds are the bolt is rusted to the captured nut. You can remove the body with the shaft of that bolt still there. Once the body is off use some penetrating fluid then use a pneumatic air hammer at the bottom shaft of the bolt. With the fluid and air hammer vibrating the shaft and nut it may break free as you can not use flame in that area.
There is an induction heating coil used to free a rusted nut and bolt. But odds are that nut and bolt are ceased together and will need to replace the caged nut assembly. That bolt most likely will look like this.
To remove that bolt, just smack it straight up with a hammer. That cage will pop free. Remove the broken bits from the top. Replace later with a washer and nyloc nut. You don't need that stupid cage.
Before you start swinging a BFH, keep in mind the cage is riveted to fiberglass so you’ll end up damaging the fiberglass. Once the body is off the frame, you can drill the aluminum rivets out easily, preserving the ‘glass.
The right rear body mounting bolt cage been removed by a prior owner on my 73.
They just put a 7/16” flat washer over the hole and tightened it up with a nut.
I wanted to add additional strength to the inside of the pocket so I cut a round piece of 1/8” aluminum which was almost the diameter of the inside of the pocket.
I drilled a 7/16” through hole in the center of the round aluminum plate and laid it in the pocket.
When I bolted the body to the frame I simply installed the bolt through the body mount, through the aluminum plate and topped it off with the flat washer, lock washer and nut.
Better and stronger than the original captured nut plate riveted to the fiberglass.
The pockets are way in the back corners, but very accessible.
I wouldn't be the slightest bit worried about smacking that bolt. Those old rivets were NEVER strong. And now there very rusty.
Most of us make up a plate to fit into the inside of that well. To reinforce it. I also poured in some marine resin with the bottom hole taped off of course for further reinforcement.
Then through drill the hole. New bolt and nyloc nut.
Sounds like you're taking the body off so once removed there will be plenty of room to access the Rivets from the bottom. They are made from Aluminum and drill very easy. You will need to center punch them accurately. We are talking about 10 minutes tops to do this. Putting in new Rivets is more challenging must for two people. Hence why somebody above used Screws. Keep in mind the Rivets have nothing to do with strength. All they do is hold the Nut, so it doesn't turn. You can use a regular Nut and Washer as said above but You will need an extra person inside the Car when You tighten it. So, it's your choice, an extra hand to Rivet, or an extra hand to Tighten. I have done this job many times. (I prefer to Rivet).
I had to grind off the remaining head of #3 body bolt, which is inside that little door, in front of the tire, with almost no access. This is the mini air grinder I used.
Remove Broken Bolt
