What grade of bolt to use on suspension and why?

The guy at Fatman said they use grade 5 for the lower control arms because they will bend before breaking. He stated the grade 8s would just snap with no bending so he said not to use those.
What I know about suspension you could fit in a thimble. What is safe and what should be used.

JD

 

I use grade 8 on anything important

 

Grade 5 works better in shear situations. Grade 8 works better in pull situations. The 5 will bend in shear before it breaks, 8 in shear will snap. Works just the opposite if you reverse the situation.

 

Grade 8. Almost all the offroad race trucks I've worked on and owned myself have used Grade 8's. They see much more abuse than my hotrod and it has grade 8's. A grade 5 will bend and break before a grade 8 reaches it's yield point.

 

I use grade 8 or L9.

Flatman

 

When i worked at the race car shop, we used grade 8 on any suspension part.

 

Grade 8.

 

Exactly.

 

Did the guy at Fatman say why their suspension bolts were breaking and bending??? Shouldn't be an issue unless it's underdesigned or you're doing wheelstands.

 

Just as Cornerfreak stated, Use Grade 8 on any critical attachment regardless if it's shear or compression. It's a misconception that a Grade 8 will shear when a Grade 5 will bend. I have always used Grade 8 bolts, washers and nuts on my cars never an issue. The FOGGER

 

A-men!!

 

I agree with most others here, I use grade 8.

 

I like Grade 8 Fine thread.........Littleman

 

I use grade 8 on anything that may get stress, my bumpers are held on with them,my clutch linkage , motor mounts etc. grade 5 I use in non stress area's like fender bolts etc.

 

When ever i can i try to use allen cap screws,they start at 80,000 psi and grade 8 are at best around 60,000 psi.May not be the most traditional but are used mostly in the aero industry,that is good enough for me.

 

I had found an article on another forum about Mustang II suspension problems and it specifically mentioned Fatmanfab. It stated that they had 6 failures of the grade 5, 1/2 by 20 by 8" bolts that attach the lower control arm to the crossmember. The truck I bought has the 1/2 inch x 20 bolts, which prompted the call to Fatman and my previous post was their response. The reasoning I was given was if its going to fail then he would rather it fail by bending first and then snapping instead of just snapping.
Sounds like from reading all these pot I need to find somewhere to by grade 8.
Fatmanfab has also switched to using 5/8 grade 5 bolts instead of 1/2.

 

Rule of thumb...Suspension, Ch***is, Power Mounting....Grade 8.

 

Keep in mind that just because the hardware store calls it grade 8 and has all the markings. color, etc. doesn't mean it's grade 8. I'd be leery of the big box hardware stores... stuff like that would be better from an industrial fastener store.

And regarding the specific bolts mentioned, when nearly the entire length of the bolt is captured, there's not much room for a bend. It's more likely to shear rather than bend anyways.

 

I remember hearing a story from a professor in one of my engineering cl***es about performing "autopsies" on jet engines that had failed during their break-in runs(at the manufacturer). A total of 2 or 3 had failed before they found the issue. Bolts had failed within the unit that had a certain grading labeled on the head. Upon testing their existing stock of these bolts, they found that the bolts were far below the grade labeled on the head. When the supplier(foreign supplier) was contacted about the problem, it was indicated that the markings on the head were for "decoration".

I'm not sure if this story is entirely true, but it did make a point.

Lets just say, this was probably before ISO certification and the like.

 

We get bolts supplied at work from Fastenal. The grade 5 bolts are slightly harder than plastercine. An L9 lists a tensile strength of 180,000 psi when used with a matching nut and washers.
Peace of mind is priceless....

Flatman

 

I was told by someone at fatman that these suspensions are for show cars only and are not supposed to be driven. I swear that is what I was told! I have been very leary of a lot of their engineering. I think I know something about engineering.

 

Grade 8 Fine Thread..
Jimmy White

 

I am suprised that someone who claims to have an engineering degree and uses that fact in his promotional literature would make a statement like this regarding bolt grades.

Take this from someone who is a self taught "engineer" with experience and practical application ranging from Aerospace, Automation, Racing and OffRoad, regular Automotive repair and construction of hot rods for 20 years now, high stress applications use grade 8 or higher. Period.

grade five is only acceptable for general purpose applications.

Proof? Stick your head under any vehicle made and count the markings on the heads of all the hardware. You will find the charicteristic six hash marks indicating grade 8 on nearly everything even specialty hardware like upper control arm attachement bolts. If it doesn't have hash marks on it you can be ***ured it is either FORGED or of a high strength material like 4340 or 8620.

 

http://www.engineershandbook.com/Tables/boltgrades.htm

And annother fantastic article, this one explained in detail.

http://www.rockcrawler.com/techreports/fasteners/index.asp

Quoted for emphasis:

 

We used to use AMERICAN grade 8 or better on our Porsche Gt3 race car. Take it from Carrol Smith (who is if you're not familiar, THE MAN), Anything less and you're risking your life.

 

grade 8 only

 

Great article, thanks!

 

The guy at Fatman is a ****ing idiot.

 

I just read an entire book about hardware by an engineer who built race cars his whole life (can't remember his name or the name of the book), and he says Grade 8 because they are stronger, period, than grade 5. The Grade 5 will bend AND break before the Grade 8 will break.

 

You sure said it better than I did! Sounds like the HAMB of lore! Priceless and correct.