Metallurgist opinion needed please

[dcdoug]

Are there any bolt stress scenarios in which Grade 5 would be stronger than Grade 8, or is Grade 8 always stronger and you are just trying to decide whether Grade 5 is enough?

[Tommy]

Originally Posted by dcdoug

Are there any bolt stress scenarios in which Grade 5 would be stronger than Grade 8, or is Grade 8 always stronger and you are just trying to decide whether Grade 5 is enough?

doug,

I'm reaching a long way back to my college days, so cut me a little slack if I don't get all of this exactly right. Engineers consider many properties when evaluating the appropriateness of a metal for a particular job. Those properties include hardness, toughness and britlleness among others. Grade 5 bolts don't have as much tensile strength (resistance to pulling) as Grade 8, but they are typically tougher and less brittle than Grade 8. So, a Grade 8 bolt is better for an application that will not be exposed to continuous on and off stresses (fatigue) and needs to be as strong as possible for occasional stresses. But a Grade 5 bolt might be better for an application where fatigue is more of a concern. So, it's not as simple as saying Grade 8 is always better than Grade 5. Given the way seat belt bolts are used, I used Grade 8 in my car.

[kitcarbp]

Take a look at these 2 articles. Seems the pros use Grade 8 fasteners. Here are two articles I found with useful info from the racing community in CIRCLE TRACK.

http://www.circletrack.com/safety/ct...ide/index.html

http://www.circletrack.com/safety/ct...unt/index.html

Based on my research so far, I am leaning towards the Schroth 4 point belts for my Cobra for street duty only. If you want to see the R&D that goes into their belts, check out their web site too, cool stuff.

Bill

[bobcowan]

This questions comes up a lot, and there's no clear cut answer.

As mentioned above, Carrol Smith's, "Nuts, Bolts, Fastners, and Plumbing Handbook" is an excellent read. You'll learn more about fastners than you thought there was to learn. He wrote in easy to understand terms, so even I could learn from it. http://www.amazon.com/Fasteners-Plum...0&sr=1-1-spell

In his book, he states that grade 8 is for clamping only. It should never be placed in shear - even double shear - because it can fracture. The 8's should only be used where there's a dowel to prevent lateral movement.

He goes as far as to say, "There is no place on a race car for a Grade 8 fastner." Not sure I agree with that statement, but there it is.

As far as the seat belt goes, I think if you manage to break a grade 5 or 8, it won't matter. The belts would have cut you in half already.