I just heard of a friend who totalled his C4 because his rear lowering bolts broke on the freeway while he was driving. The bolts where Grade 8. My other friend and I got a little paranoid and ckecked our rear lowering bolts on our C5's. I just installed some 1/2x6in grade 8 bolts and nuts so it looked new but my friend with the same bolts found a stress crack already only after about 5,000mi and less than 2yrs of life on them. I want grade 9, 5/8 bolts which should stand up to a nuke bomb but theyre hard to find indivisually. Grainger only carries the nuts and washers in boxes. Two questions: 1) Where can I find 5/8, grade 9 bolts and hardware indivisually (Im in Los Angeles)?, 2) Would grade 5, 5/8 bolts be stronger than my grade 8, 1/2in bolt since its thicker? I have these at work and can get them free. Thnx.

 

check http://www.mcmaster.com/

 

Thnx for the link.

Here's what Ive found.
Bolt Tensile Strength, for coarse thread (Breaking Strength -lbs)
Bolt Dia. Grade 5 Grade 8
1/2in. 17,000lbs 21,300lbs
5/8in. 27,100lbs 33,900lbs


As you can see, a grade 5, 5/8 bolt is so thick it is much stronger than a 1/2in grade 8 by 5,800lbs of force. I doubt that the leaf spring will ever see or exceed 27,100lbs of force on a 3,200lbs car. Still, guys, upgrade those bolts!

 

i just used the stock bolts and cut the bushings, no need to buy anything.

 

Grade 5 is weaker than grade 8, but the additional increase in size to 5/8" might be of benefit.
You'll have better adjustment with fine thread bolts rather than coarse thread bolts.
Some suggest using 9/16" in grade 8 instead of 1/2" in grade 8. Whatever you elect to use, do not buy them from a home improvement center. Buy them from a fastener specific vendor. Ask them if they're US made or off shore made. MIL-SPEC might be a good choice.
You're going to need a metallurgist or fastener "engineer" to give you a definitive answer. Maybe McMASTER-CARR has a technical desk to assist you.
The oem rear bolts are 14mm, but no one apparently supplies 14mm bolts in 6-7-8" lengths. Maybe it's for a good reason. Any fastener will have it's breaking point.
I don't know what the oem bolt grade is. It should state on the head of the oem bolts.
The suspension is under plenty of stress, especially on typical street driven vehicles.
I always cringe when I see someone reports their after market rear bolts failed. I'd be willing to bet there are plenty of grade 5 or grade 8 bolts with stress cracks with inevitable failure in their future. And yet 1/2" is typically what is suggested by owners and/or sold by vendors. How many times do you hear the oem length bolts failed?
Some members have previously reported their after market or hardware store bolts have failed and damaged their suspension or wheels.
Maybe coil overs would be a better option if you consider the potential for disaster.

 

9/16 grade 8 is a little stronger than a 5/8 grade 5 by about 1,900 clamp load lbs. Not sure on the tensile strength but I know that the standard automotive grade hardware is grade 5. The stock bolt looks pretty wimpy too so I really hope that these 5/8 bolts will be more than strong enough. 5/8 grade 5 is almost as strong as a 1/2 grade 9.