First, set aside the used bolts that you cannot identify.

1.Carry the rods to the place you buy graded bolts from and
get the highest grade possible that will fit; usually grade 8 or metric 10.9.

or 2. Call the rod mfg'r and ask them.
or 3. Call ARP and ask what they recommend.........but know the thread count per inch before you call.

If you need help with identifying what you have or need, bring them by and I can help you. I don't like to see someone mess up
their equipment...........and the wrong bolts could very well do that-Bob