Basically the strength of a bolt is determined by its grade. For metric bolts the grades are ISO numbers as described below. For inch bolts the numbers are SAE bolt grades which run from 1 to 8.2 (the numbers have no intrinsic meaning that I have been able to determine).
A grade 5 bolt is a normal hardware store bolt while a grade 8 is a much higher strength item. The grade 8 would be about 50% stronger and would thus be much less likely to twist off or break. I have never actually seen a grade 8.2 so don't bother looking for one.
You can tell what grade the bolt is by the markings on the head. The grade 5 bolt will have 3 lines engraved into the head which run radially out from the centre. The grade 8 will have 6 lines. More lines = stronger bolt. Grades below 5 are essentially junk and shouldn't be used on anything - they have no lines.
Metric bolts run from ISO grade 4.6 to 12.9. You would be looking for a grade 9.8 or higher. The grade numbers are actually engraved on the head of the bolt.
Now as for materials - I don't think that stainless is such a good idea unless you can find one of a high enough grade. Stainless is funny stuff and some of the bolts which are sold as stainless may actually only be plated. The problem is that some type of plating are extremely bad in a fatigue loading situation such as one gets in a car. The plating (especially chrome plating) causes metallurgical changes in the base metal that make it prone to cracking. Cadmium plating is good but I don't know how it stands up to high temperatures.
I would buy a can of anti-seize compound and coat the bolts with that and just install the highest grade bolts you can. The stuff I have comes in a grey coloured plastic bottle and it has brush built into the lid. The compound is silvery in colour and is pretty goopy stuff. But it really works and doesn't seem much bothered by high temperatures.
To get the old stud out I would buy a thing called an easy-out which is basically looks like a tapered bolt with a left handed thread. You drill a hole into the end of the existing bolt or stud and then you thread the easy-out in and keep turning. The fact that it has a left handed thread makes it get tighter and tighter as it screws its way in until (hopefully) the stud comes out. Some heat from a propane torch can help here as well.
The metric grade 9.8 bolts have a minimum proof strength of 650 MPa (just under 100 ksi) which means that they should be pretty tough and hard (which aren't really the same things, by the way.)
The SAE grade 8 bolts have a minimum proof strength of 120 ksi so they are somewhat stronger than the metrics.
If you tell me what diameter the metric bolts are and what the length of the grip is (grip = length of clamped material between the bolt head and the nut or tapped hole into which the bolt is threaded) I can calculate what the torque should be for the metric bolts.
You are quite correct, there is only one grade stronger than the SAE Gr. 8 but there are actually 2 grades stronger than the metric 9.8. These are the metric 10.9 and the 12.9 but they are likely very hard to find and not worth the extra money that they undoubtedly cost for this application.
On the subject of shearing off the bolts - sometimes very strong materials are susceptable to stress corrosion cracking (SCC) and this may be what happened to the bolts on your front end. SCC takes a while to fail a part and this may explain the delayed failure.
It is unlikely that they would have been sheared off when installed since they would likely make a loud "crack" when they failed and the installer couldn't miss that too easily.
When you install the new ones, I suggest that you put some anti-seize coumpound on them to help seal out water and crap from the area underneath the bolt head which is where they usually fail.
Do not substitute lower grade fasteners on suspension parts.
From: Dr. Peter Frise
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