Originally Posted by eljefino
^ I like the science behind this-- it tickles my nerdy senses. But if that clamping force were all it took, a giant, appropriately torqued c-clamp around the subframe, holding the bushing in place with friction, would be all we need. But that's a recklessly goofy way to build a car. What if OP hit a curb, and a shock load went through the control arm and rubber, jarring that bushing within the frame pocket? That extra .97mm, on top of the nominal (.25mm?) clearance for ease of assembly, would not be good news.
Clamping force is what a bolt delivers when torqued. If the bolt loosens up then double shear is at play and yes in this regard the m12 is 11% less. minimum double shear load of 1/2 bolt loaded through the shank is about 34k lbs. So 30k lbs for the m12 which is more than adequate.
The load the bolt will see is dependent on the joint stiffness ratio, which means it's much less than the shock load received by the control arm.
No the bolt/bushings isn't moving around if torqued appropriately, once a bolt is torqued the load is carried by the bushings metal sleeve via friction and clamp load.
Long answer that the m12 is likely fine. OEM is the safe answer though.