The issue with grade 5 bolts, is that they are to soft to torque up to the extreme force it takes to clamp headers. The mild steel that the header flange and tubes are made of expand and contract much more that cast iron exhaust manifolds do, that have very little expandtion a contraction. This warrants extreme clamping force to get and keep a good seal that is not needed on a cast iron manifold. Also chances are, your going to have to replace header gaskets every few years. About 2 to 3 years is about as long as I get out of my gaskets, and I have tried every brand header gaskets they make. My best results have been fel-pro header gaskets with over torqued bolts. I have rounded over the small grade 5 header bolt heads torquing or removing them. Again as I said before, I drive my truck every day. This does help the rust factor. As for a stuck bolt, I deal with that almost every day at the shop. I weld a nut over the broken stud. Once the nut cools off, 90 percent of the time, the heat from welding loosens the stuck bolt once it has cooled off, and is easy to remove. The other 10 percent of the time, the bolt has to be machined out of the hole, or in this case, yes it would need to be drilled and have the threads chased, but I would use heat on the head around the bolt hole first. We all have our own way of doing things, and I am a welder after all, so I use what I know. Only giving my logic on why I choose grade 8 header bolts over grade 5. Also I have had to use vise grips to remove rounder over heads on grade 5 header bolts to remove them. Never on grade 8 header bolts.I have to respectfully disagree with grade 8 or hardened bolts in exhausts. If it rusts through and breaks, you're going to have a devil of a time extracting it. As I said in another post, feel free to ask my how I know. The clamping force necessary to attach your exhaust is easily within the limits of grade 5 hardware.
I wonder if a lot of the issues with header bolts breaking is because people improperly use exhaust hangars. You don't want the exhaust swinging around, but you do want some give in the hangars to allow for engine torque. If a sideways force is repeatedly applied to the bolts they won't last long, even if they are grade 8.
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