[blykins]

Guys....

I've researched a little bit since the post started earlier. I was kinda confused about some comments that Dan Case made. He made the comment that Grade 8 bolts were more brittle than Grade 5....and that Grade 5 bolts would just plastically deform rather than suddenly fail.

I took his post, but didn't really put much thought into it at the time....knowing the fact that materials can be worked in different ways, giving them different properties and failure modes.

However since then, I've found some interesting facts....

Grade 5 bolt = 120000 psi TS (tensile strength), 75000 psi SS (shear strength), Rockwell Hardness C25-C34.

Grade 8 bolt = 150000 psi TS, 91000 psi SS, RH C33-C39.

Now everyone knows that there are tolerances when manufacturing takes place. But given that you have one of each Grade bolt and they each spec out perfectly to their design specs, then there is only one way to rule this one....

Grade 8 is superior overall.

It will take 30000 psi more in tensile stress and 16000 psi more in shear stress. That's a lot.

Tensile strength pertains to the bolt being pulled axially (fixed on each end, trying to pull it apart) or it could pertain to the bolt being bent.

Shear strength could be taken in two ways....vertical shear (similar to laying a log down and chopping it with an axe) and torsional shear (grabbing a piece of round metal and twisting each end in opposite direction).

I saw that Grade 5 stuff is made from mild steel. Grade 8 is made from mild alloy steel. That's why Grade 8 is a little harder.

Which one to use? To answer that question for each part of the car, you would have to do a statics analysis and fatigue stress test for each scenario. Some studies would probably show that a Grade 8 bolt would be overkill.....

But this is how I look at it....What does a bolt do? It essentially clamps two pieces of metal together and holds it. You have the head of the bolt pulling the fastener in one direction and you have the nut pulling the other direction. This is axial tensile stress. Unless you do a study on your application and find the tensile stress is not that high, then I would overkill the situation. Better safe than sorry.

Why do some manufacturers use G5 stuff over G8? Because the loads are not that great...and it's cheaper.

Even if it were the case that a G8 bolt would just fail all of a sudden while a G5 bolt slowly plastically deformed.....I would still want the G8 bolt. Why? Because of the tensile strengths of each fastener. The G5 bolt would be already deformed and stretched out 30ksi earlier.....The G8 bolt would still be going strong in its original shape at that point.