JamesDowning wrote:Careful - Grade 8 are stronger, but also more brittle.
If GM specifies grade 5, it's probably because they "stretch" better, which is needed in high-heat variation areas like headers. Your bolts have to expand and contract properly with the header flange. This is one place where I would probably buy genuine GM parts or other bolts specified for headers.
I'm not sure that your explanation is precisely true. The level at which a grade 8 bolt shatters is much higher than the grade 5 under any conditions. The tensile strength of a grade 5 bolt is typically 120,000 pounds per square inch, while grade 8 bolts are typically spec'd at 150,000 pounds per square inch. Both are "ductile" meaning that they will deform under pressure and not just snap (brittle). That would not be useful for bolts, especially ones designed to hold superior load limits under either ductile or shear strength conditions. There are some bolts that are not ductile, but they are not commonly found in automotive use.
The information above was gleaned from
Mark’s Standard Handbook for Mechanical Engineers.
Grade 8 bolt capability in yield (stretch) = 130,000 lbs / in2 x .03349 in2 = 4354 lbs minimum
Grade 8 bolt capability in tension (failure) = 150,000 lbs / in2 x .03349 in2 = 5024 lbs minimum
Grade 5 bolt capability in yield (stretch) = 92,000 lbs / in2 x .03349 in2 = 3081 lbs minimum
Grade 5 bolt capability in tension (failure) = 120,000 lbs / in2 x .03349 in2 = 4019 lbs minimum
The grade 5 bolt has already broken before the grade 8 bolt even begins to deform.
Additionally, Grade 5’s typically have a core Rockwell hardness of C25-C34 whereas a grade 8 typically has a core Rockwell hardness of C33-C39. Grade 8 are tougher and more impervious to outside forces, such as the shear of a winch plate or tow point bracket.
There are likely other factors at stake for the exhaust manifold bolts, however, that make Jame's suggestion of using factory parts valid. Bolt shoulders, intended stretch, etc., all factor in, but that being said, I've run a lot of grade 8 hardware on exhaust systems including headers over the years.