i would have thought to go grade 8 also, but i read on some thread here that a guy with a C5 was driving around and one of his bolts snapped. he went somewhere and they said it was because grade 8 bolts are only made to hold tourque and are brittle when it comes to flexing. but i would assume this is a very rare occasion when it could have been a bad bolt, that just snapped.
http://www.digitalcorvettes.com/forums/showthread.php?t=72539&highlight=bolt+snapped
heres the link
I had seen that thread before and stopped reading after the first page. Everyone saying to use grade 8 was correct. Not sure why the fastener store guys said grade 5 would be better. :huh:
That guy's bolt has an obvious flaw in it, as was pointed out by the people posting about the area of rust next to the area of the clean break.
Effectively all of the weight of the rear end is suspended by those two bolts, and they are only in tension. All that BS about flexing is irrelevant. The bushings ensure that even as you're going around a curve with lots of lateral acceleration the bolt is still only in tension. The strut rods and halfshafts will undergo more strain during lateral acceleration, but those bolts will only feel a slight fluctuation in stress.
Although I would expect that the 92,000 psi yield strength of a grade 5 would suffice, you might as well go with the 130,000 psi yield of grade 8 for a higher safety factor. Grade 8 will probably come with some sort of rust preventative coating (probably zinc) that will help prevent failure due to corrosion.
The lowering kits that VB&P sells have 8" bolts:
http://www.vbandp.com/SearchResult.aspx?CategoryID=275
I believe these are 1/2" diameter, but you should compare them to the bolts you pull out of your current spring. Fastenal has 8-1/2" long by 1/2" diameter zinc coated grade 8 fine thread bolts for $5:
http://www.fastenal.com/web/products/detail.ex?sku=18931
If you're looking to go as cheap as possible just pick those up, along with some nyloc nuts and you'll be a bit lower.