TeamTruett

On the differential cover-to-crossmemeber and the cross member cushions I used grade 8 bolts. I think originally they are grade five, not sure. I was told that grade 8 bolts are more brittle than grade 5 and are not always the best choice for certain componets.

vintagecorvette

Grade 8 bolts are stronger than Grade 5 bolts.

No problem using them in the dif cover or crossmember.

oWEN

Plasticman

Yes they are stronger (higher tensile strength), but are more brittle (as you noted), and more prone to fracture.

Recommend in high vibration prone areas to use the correct grade bolts. I don't know if that applies to your questioned area, but just a caution.

Plasticman

project63

Bolts Grade 5 - Structural bolts identified by 3 lines on the head. Grade 5 bolts can withstand 120,000 lbs of tension per square inch of cross sectional area.

Bolts Grade 8 - Structural bolts identified by 6 lines on the head. Grade 8 bolts can withstand 150,000 lbs of tension per square inch of cross sectional area.


I don't know any location on an automobile where the grade 8 bolts would be more prone to failure. After all, I think they are specified to bolt the connecting rods to the crank...how much more demanding is any other application on your car?

tc

Plasticman

See paragraph from Engine Builder magazine article here:

“One of the issues with Grade 8 bolts is that there are some areas where you really don’t want to use them,” says Doc Hammett, Totally Stainless. “If there’s a cycling load on them you could start to get into trouble. A classic example was on the old belt drives where street rodders were using Grade 8 for accessories and they were breaking bolts all of a sudden. Many were left scratching their heads until someone figured out the bolts were fatigued. The higher the carbon steel the more they are prone to fatigue. Fastener manufacturers add other alloys to carbon steels and change the properties to suit their specific needs. This is one of the most interesting things about steel: you can add a little bit of something and make the properties change drastically.”

See entire article here:
http://www.enginebuildermag.com/Arti...ner_facts.aspx

Plasticman

Mikey65

What about suspension components then? Seems like there would be a lot of vibration there, yet companies like VBP, VANSTEEL and many others use grade 8 for just about everything.

GCD1962

I seriously doubt the way most everyone drives their cars that there would be any problem using a grade 8 bolt anywhere on the car

63 340HP

The only way the fastener (bolt, stud, etc.) can have cyclic loading is if the mated parts and fastener are moving in relation to each other. This is highly unusual in most applications.

The common areas that are a concern are head bolts or studs under constant compression cycling, and thermal cycling of the exhaust manifold bolts. The key element is the mated parts have an external influence that causes cycling of the fastener (the chamber pressure, and the dissimilar thermal expansion rates of the manifold casting and fastener). Care needs to be observed to assure the cycling displacement remains below the fatigue limit of the fastener (why the NASA Engineers and ARP folks get the big $$$, they sweat the details where needed).

If both mated parts of the assembly vibrate together, there is no problem using a high grade/strength bolt, just remember the surrounding weak link will remain (the sheet metal or aluminum the assembly is attached to). Care also needs to be taken when working aluminum and titanium (and other dissimilar metals) to prevent stress concentrations from scoring and/or the need for a dielectric barrier to resist electrolysis. There are books on fasteners if references are needed (good insomnia combat reading ).

Good Luck

jim lockwood

One of the problems I've encountered with bolts that are supposed to be "Grade 8" is that they have inconsistent bolt to bolt properties. I first encountered this when I was planning to safety wire key fasteners on my vintage racer. I found my drill could zip through some "grade 8" bolts and barely make a dent in others, all out of the same box.

And then I looked more closely at the box.

Turns out that it was impossible to determine the country of origin of the bolts I was using. They could have been US made or they could have come from an unnamed country that was recently caught sending contaminated baby formula to the US. Or they could have come from somewhere else.

Soooooo...... I changed my bolt strategy completely. I switched to using aircraft bolts, nuts, and washers. Not as strong as grade 8, but entirely consistent from bolt to bolt. For situations where I believe brute strength is the right solution, I buy ARP bolts, nuts, and washers.

That's my strategy and I'm stickin' to it.

Jim

macdarren

I almost always use G8 bolts...I have never seen an instance where they were provably inferior to G5 in any automotive application...and they are must less prone to being twisted off by a little too aggressive hand tool efforts... especially true when the bolts are a bit frozen after a few years of service. I don't think they are significantly more 'brittle' but they are very noticeably less likely to be twisted or bent....plus they are are generally treated to a nicer finish if you don't like a bit of gold tinting. FWIW most chromed bolts are also G8....BTW: if you ever opt to do chrome fasteners be sure to run a tap or die as appropriate as they tend to be tight after chroming and will seize permanently if with very little effort (learned this the hard way). The dangerous stuff is the SS bolts....they are often just barely G5 and I find that they are more brittle....but they do look nice and stay corrosion free so for non critical, low load places I like them.

And I agree with Jim and 63 340Hp on the fatique issues and the use of ARP were needed.

AZDoug

G8 bolts aren't necessarily more brittle/inferior/prone to arbitrary breakage, what you are seeing is the difference between 2% tensile yield strength vs ultimate tensile strength.

2% yield strength is the force it takes to make the test sample elongate 2%. Ultimate tensile is the force applied at which time the sample actually breaks in tension after it is stretched beyond it ability to flow.

A bolt with say a 120 KPSI 2% yield strength and an ultimate tensile of 160 KPSI will stretch a lot before it breaks, a bolt with a 180 KPSI 2% yield strength and a 190 KPIA ultimate tensile strength will seem to snap, as it doesn't stretch, thus refered to as brittle.

Doug

RobRace10

Most race car teams and builders use AN bolts not grade 8 for any critical applications because the grade 8 are prone to fracture. I doubt we have a single grade 8 bolt in our collection.

JohnZ

They'll work fine in your application - don't worry about it.

Metal/corvette

I found out the hard way not to use grade 8 bolts on exhaust manifolds ,they don't strech with heat expansion.
Broke quite a few bolts untill I went back to soft bolts made for exhaust manifolds.

LPlum

I used all grade eight bolts, for the exact locations you are describing. Many years ago, and have had no problems.
Lowell

obx2323

That's not a high torgue and no movement location (it's gives and is dampened, not rigid) Suspension has other areas to displace the suttle consistent vibration where as a solid metal to metal and twisting (torgue) will speed metal fatigue and = stress cracks= failure.

TeamTruett

Thanks for all the replies, looks like Im OK