[ItBites]

The best fasteners for exhaust that I have used are made of A-286 stainless steel. Do not use the 300-series stainless steels as these are not only not very heat resistant, they are also very weak. I really like the NAS1351N - series fasteners for this type of application, since they are designed for lap-joint applications and have a long shank and short threads (threads are not in line with the split). Pegasus may sell them or you could try McMaster-Carr, but I usually get them from the local fastener distributor (Copper State Nut and Bolt). They are not cheap and often have to be ordered.

Also, I agree you likely have higher than normal temps (timing or mixture), but if you go to A-286, they won't get brittle...