If your quad is anything like mine then it has seen a hard life. I purchased mine as a used fixer upper and it had seen plenty of mud. So much in fact that it had made its way into everything. Every nut, brake pad, bearing, etc.
At this point it seems hopeless but you reach for that as seen on TV penetrating fluid that has never let you down and blast it. You even let it sit for 24 hours and still it won't budge. Then you get the torch. After plenty of heat and smoke from the penetrating fluid it still won't budge. You guess it is time to break out the drill bits and taps right? Well hold on there. There may be hope yet. A simple trick just may save you before you give up. Machinists know this one well and it won't cost you much at all. Take a ride down to the local dollar store armed with a buck and purchase a few candles. I doesn't particularly matter what kind, but if lavender scent suits your fancy then go for it.
The trick here is Paraffin. While most penetrating lubricants contain paraffins they do not contain them in the concentrations that a candle does. Why? Because obviously in the concentrations a candle has them, they are solid at room temperature. Rather they are mixed with lighter petroleum distillates. In most cases these rust busters work fine. The lighter petroleum distillates not only lubricate and creep through rust but allow the paraffins to flow into the crevices and work their magic.
So now that you have the candle what do you do. Return to the torch that is what. Once again heat the affected area up. Be sure to apply heat as evenly as possible around the entire affected area but don't stray far from the fastener. A lower temperature setting and extra time is recommended here. You want everything heated thoroughly and the heat to go the depth of the fastener. Once you think you have it apply the candle directly around the head of the bolt, hole, or threads. Allowing the wax to be drawn into the threads. Reapply heat and repeat the waxing process. After a couple applications of wax attempt to remove the bolt but don't risk breaking it. Hopefully it goes but if it doesn't repeat the process. Be sure to clean the threads afterwards.
When all else has failed this has always came through for me. Best of all I can find three packs of candles at the local dollar mart for a buck and they out perform the best penetrating fluids.