Interesting Discussion.
Not sure that I aggree with some of the comments being made.
"The reason the grade 12 bolts failed was the lack of elongation of this material. The correct torque setting is very near the yield point of the steel. Any extra loads induced by spring articulation etc will likely cause the bolt to fail. As I think you will know, you cant get around this by using less torque on the bolts as they will have other issues like undoing or shockloadings happening due to not enough clamping force."
This dosen't quite make sense to me.
All ferritic steels (mild, high tensile, 4340, 1030, spring etc) have virtually the same stretchiness or elasticity (youngs modulus) ie load versus elongation, its just that mild steel bolt will stretch less before it starts to yeild than say a high tensile bolt. That is why mild steel is no good as a spring while a hardened and tempered high carbon steel is.
The clamping force generated by a bolt and nut is not dependent on the material, it is dependent on the torque, thread pitch, surface finish and effectiveness of the lubrication.
You can tighten a high tensile bolt to the same torque as a mild steel bolt it that torque will provide the clamping force required. If you then have the situation where the bolt is put under more load due to external forces, if the load goes above the yield point of the mild steel bolts then the assembly will become loose and may even fail if it goes above the UTS (ultimate tensile strength) wheresas with the high tensile bolt it will stetch and then return if the load dosen't go over the yield point, if it is severly overload then yes it will also fail.
Yes I agree that the amount of stretch that will occur from the point of yeild to the point of failure is less for a high tensile material, but it takes alot more load to get to that point.
And yes very hard materials are more brittle when impacted from an instantaneous shock, but I would say that the type of shocks that occur while driving do not give an instantaneous type shock.
This is the reason that for tow hooks it is required that you use high tensile bolts and not mild steel.
Personally I would use Grade 8.8 as it gives good strength while still yielding a bit before they fail. But it is up to you what you use.
For those that don't know how metric grades work.
4.6 means 400 Mpa UTS and 240 Mpa Yeild
8.8 means 800 Mpa UTS and 640 Mpa Yeild
10.9 means 1000 Mpa UTS and 900 Mpa Yeild
12.9 means 1200 Mpa UTS and 1100 Mpa Yeild
These are approx values.
As a comparison for an M12 x 1.75 bolt.
A 4.6 will yield at about 1.8 tonnes and fail at about 3.3 tonnes load.
An 8.8 will yield at about 4.8 tonnes and fail at about 6.6 tonnes load.
If you use a metric fine ie M12 x 1.25 then they will be stronger.
Hope this is of some help.
Regards
Tim
) I've converted them to bolt mounts instead of the U bolts, grade 12 caps 
Holy shamoley 
but I"ve already got some thing underway to allow the movement and it won't require custom springs with larger eye's 
still uncertain how hard I neg flex the springs and how big to make the bump stop once I make my mind up I'll bolt a set on
