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I'm setting up a motorcycle front end from used parts, and some of the mounting bolts were not included. The same bolt is used both for mounting the calipers to the forks, and mounting the brake discs to the wheel hub. I've found the size and part number (M8 x 30mm) that is used on the OEM parts, but it does not indicate what material or grade of bolt to use.

Stainless seems like a good option because of corrosion protection and I was able to find these in an A2 alloy, but I'm concerned because of the considerable shear force that may be put on the disc mounts. FYI The torque spec is ~20 ft lbs.

Will these A2 bolts be sufficient? Or Should I go with something stronger, say a carbon steel 12.9 grade bolt?

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No, A2 bolt is not safe. You can use the calculator here: http://www.tribology-abc.com/calculators/e3_6b.htm ...for bolts of grade 8.8 and M8 thread. It will say that the maximum tightening torque is 24.11 Nm which is less than 20 ft lbs (which would be 27.11 Nm).

Now, the calculator does not have A2 as the grade of bolts, but according to this site: http://www.volksbolts.com/faq/basics.htm ...bolts of grade 8.8 have 0.2% yield strength of 635 N/mm2, whereas A2-70 has 0.2% yield strength of 450 N/mm2. So, A2-70 is even worse than 8.8 which was found unacceptable for this application. Note that even A4-80 would be unacceptable, being also worse than 8.8. So, I would use carbon steel 12.9 grade bolt.

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  • Thanks, suspected I should probably go with high grade carbon steel but it's good to see the proof.
    – Christopher Hunter
    Mar 16, 2017 at 19:18

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