in addition to what Frank said, there also can be a difference in the stud and nut size. Many of the 3" ones I use are small 1/4" diameter studs and nuts, and I have had those pull the base of the stud out of the collar that holds it in. The ones I've preferred are the larger 5/16" studs and nuts, never hurt one of those. They look identical other than stud size.
I am probably over-tightening them, don't know. It isn't the stud and nut that fails, it's the little collar that holds the stud to the clamps. The larger ones just seem to be stronger there. The ones that lasted for me were from Dynatech.
http://store.summitracing.com/partde...5&autoview=sku
Jody