It’s a good idea to use mounting kits designed for a specific application when selecting hardware to mount your manifolds, risers, and elbows. Mounting kits available from Sierra or the engine manufacturer will include the appropriate bolts.
If this is not available or you’re buying bolts in bulk, you generally can’t go wrong with grade-8 or grade-5 zinc-plated bolts, as long as you’re working with cast iron. It can sometimes be less-expensive to buy bolts in bulk from a bolt supplier, especially if you don’t need gaskets.
If you’re working with stainless steel or aluminum, or have aluminum heads, please refer to your engine manufacturer as improper hardware or tightening can lead to damaged threads or cracks.
Please refer to this chart for bolt tightening recommendations. This is for common cast iron applications only.