[eschaider]

A socket head cap screw is usually class 14.9 in metric talk with a tensile strength equal to or greater than 180,000 psi. This is well north of an SAE grade 8 fastener.

If it is stainless then it is likely at 70,000 psi tensile or lower and probably a Metric class 5.8 (or lower) fastener which is comparable to a grade 3 or 5 SAE fastener and does not belong where you are putting it.

If you use a metric hex head fastener from your local auto parts or hardware store you are likely down in the class three or five category unless you can find a fastener that is marked as class 12.9. If it's head is marked as class 12.9 then it is approximately the equivalent of an SAE grade 8 fastener with 170,000 psi tensile strength. These are not typically in your local retail store.

For safety and longevity reasons you should be using a Metric black oxide steel cap screw. These fasteners will minimally be class 12.9 and possibly class 14.9. The hex head fastener in your picture is a dangerous choice.

When you select your replacement fasteners pick a fastener that has unthreaded shank in the load bearing locations and tightening threads outside of the load bearing locations. You will likely need to use hardened washers to properly position and tighten the fastener.

While I don't know with certainty, I strongly suspect the reason for your fastener failure was the threads on the broken fastener were placed in shear. This is a big no-no. Use the unthreaded portion of the fastener for load bearing purposes like shear and the threaded portion of the fastener for applying tensile load to tension and lock the fastener in place.

You can find the fastener(s) you need at McMaster-Carr or Belmetric.