Bolt for Coilover....

A grade 8 bolt for a coilover to a lower control arm is plenty tough...right?

Depends.

Is it a metric bolt or an SAE bolt? The grades are different. An 8.8 metric is only about as good as a Grade 5 SAE. Your metric equivalent to an SAE grade 8 is at least grade 10.9.

https://www.boltdepot.com/fastener-information/materials-and-grades/bolt-grade-chart.aspx

Remember, the load on the bolts is the preload from tightening it PLUS the load from whatever forces are acting on what it's clamping, and in a suspension part that you need to take apart over and over again, you want that total load below the proof load in the chart.

You can use this calculator to figure out what your clamping load is:
http://www.engineersedge.com/calculators/torque_calc.htm

So if you have a 3/8" steel bolt threaded into steel at 35 ft-lbs, you'll have a 5,560 psi clamp load, which is going to be about what the tension on the bolt will be. Going back to the first chart, on an SAE Grade 8 bolt, you'll have 115,000 PSI of proof load left to impart before the bolt deforms.

Now, how much force is exerted on a coil-over mount? I don't know, but we can try and figure it out.

F=ma. My car has 930 pounds (421.8kg) sitting on a front corner. So that's 421.8 * 9.82m/s^2 = 4,142 Newtons of force for every second the car is traveling towards the ground. We'll assume a 1 second vertical descent just keep the numbers simple.
Now we need to know the surface area that force is acting on to get the actual load on the bolt. We'll assume a 12mm fastener to keep it metric.
Area = 2*pi*r^2, so 2*3.141*6^2 = 2*3.141*36 = 226.19mm^2
4142N acting on 226.16 mm^2 is 1,832.74N/cm^2, which a handy conversion calculator tells me is 2,658 psi. And that's without subtracting energy absorbed and dispersed by the tire, and assumes the entire load is handled by one bolt. If you have two in there, cut the load in half.

That's well within the tensile ratings of Grade 5 SAE and Grade 8.8 Metric.

So yes. A grade 8 bolt should be fine for a lower coil-over mount.

PS: I just did this right now to see if I could figure it out for myself. Not trying to be a smartypants. And I might be completely wrong in how I went about this.
 
I went grade 8
A trip to fastenal took care of it

Made sure everything on the ass end is grade 8
 
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