Swingaway Bolt Extraction
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These bolts are notorious for breaking, due to the constant exposure to dirt thrown into the quarter panel by the rear tire, and water coming in around the taillight. Fortunately, these both broke inside the swingaway bracket, leaving some exposed bolt to work on.
1) Use a light BP hammer to vibrate the bolt so the penetrating oil can work into the threads from both ends. Wire-brush the inside threads as clean as possible;
2) Grip the exposed stub as tightly as possible, and gently but firmly rock it back & forth to determine how badly it might be seized. If movement is detectable, add penetrating oil and slowly increase the distance the stub is unscrewed;
3) If necessary, use a torch to heat the nut inside the body, being careful not to scorch the wiring (Aluminum foil is effective at protecting it) or the paint;
4) When the bolt loosens, keep its threads wet with penetrant while unscrewing it.