When to use Grade 8 bolts

[Alpine 1789]

This question is related to my front suspension work but I decided to make it a separate post since it has broader applicability. This is something I have always wondered about and have searched the web for answers, but not found anything definitive. The question is "when does it pay to use Grade 8 bolts?"

Let's take my front suspension as an example. I need to put the lower ball joints in. There I can use both a Grade 8 bolt and a Grade 8 lock nut to hold the joint in place. This one seems like a no brainer to me, but maybe not. Should I go with Grade 8, buy new Grade 5 or just reuse the stock ones?

Now, on to the suspension itself and the 4 bolts that hold it onto the chassis.
Grade 8, Grade 5 or old? I assume the problem here is that the bolts are being screwed into the factory threads in the frame, which are probably Grade 5 at best, but wouldn't a stronger bolt still be better?

Part of my new vs. old question is based on the belief that Grade 5 bolts were better made in 1966 than the ones you get today at Home Depot (or, in my case, Ace Hardware), but I have absolutely nothing to back up that assumption.

So, any advice on this topic will be much appreciated. Like I said, I have wondered about this for a long time.

 

[Bill Blue]

Jim, I am not afraid to reuse stock bolts that are in good shape or new Grade 8. But you know my reputation. Use Grade 8 when failure would be life threatening. Say the suspension parting company with the either the frame of parts of itself.

If you are serious about not experiencing bolt failure, use cap screws. They are 10% (or more) stronger than Grade 8.

Bill

 

[tom o]

Here is what I know about bolts. Grade 5 bolts have a tensile strength of 125,000 PSI. That is a 1 inch dia. bolt will break if tensioned beyond that weight. A grade 8 bolt has a strength of 150,000 PSi. If the bolt is made under SAE standards. The shear strength also goes up but I don't have those numbers. The higher the grade of bolt makes it harder, and more suseptable to breaking under shock, like a bearing race. Grade 5 is likely what our cars had originally.
Tom o

 

[DanR]

Jim, Clean the head of the bolts for the crossmember and take agood look! It should have codes that you can cross reference. The Rootes parts book and provide the correct bolt, screw, nuts, etc., etc.

I used my old bolts a, screw and nuts if they were able to be cleaned and had good threads.

 

[jumpinjan]

I use new grade 8 everywhere (bolts/nuts/washers/lock washers and so on) even the 1/4-28 x 1/2" for the bonnet & boot fasteners. Also, buy them by the bag in 100s at Fastenal. If I find lesser grade on a car I'm working on, I pitch them in the trash.
https://www.fastenal.com/web/home
Jan

 

[Alpine 1789]

I use new grade 8 everywhere (bolts/nuts/washers/lock washers and so on) even the 1/4-28 x 1/2" for the bonnet & boot fasteners. Also, buy them by the bag in 100s at Fastenal. If I find lesser grade on a car I'm working on, I pitch them in the trash.
https://www.fastenal.com/web/home
Jan

What about cases where the bolt is going into a captured nut or threaded area on the car, as would be the case with the suspension mounting bolts? Do you worry when a Grade 8 bolt is going into a softer thread?

 

[65beam]

bolts

jim,
when doug rebuilt the last two front suspensions for me he used grade 8 bolts. we have one harrington in the shop now that we're not sure what we will have to do to mount the cross member since someone dropped a bolt from the top thru the tube on the frame rail for the back two cross member bolts and then welded the bolt to the frame rail. at least you don't have to contend with something like this.

 

[Alpine 1789]

Thanks, Bob and Jan. I may not use 100% Grade 8, but will use as many as I can. After all, now that the suspension is going to be the best looking part of my car, I might as well make it the strongest too.

 

[pcmenten]

I have heard of the best race shops that won't allow a Grade 8 bolt in their shop. Grade 8 bolts are not the right bolt for anything except flanges. In some situations a Grade 2 bolt is the right bolt, such as bolting a bellhousing to the engine.

Grade 8 bolts don't stretch very much. For a bolt to work right it needs to stretch, it needs to be in its elastic range. If it is not in its elastic range, it is either too loose and not doing its job, or too tight and close to failure.

A Grade 8 bolts have a lower fatigue life than Grade 5 bolts. There's an account of a high vibration application where the Grade 8 bolts kept breaking. They installed Grade 5 bolts and the bolts didn't break.

Good luck to you.

 

[Bill Blue]

A person can hear a lot. Unfortunately, a sizeable portion of it is pure rubbish. The Grade 8 vs. Grade 5 debate is one of them. Here is some real information on the issue.

http://tinelok.com/grade-5-vs-grade-8-fasteners/

I think the pitfall in the argument is the assumption the same steel is used in both grades, the only difference being heat treatment. If that were true, the Grade 5 would be best for some common applications. But the same alloy isn't used and the Grade 5 is almost never the better choice.

Bill

 

[George Coleman]

I used grade 8 on my Tiger and Alpine in the front and where I thought it needed it but remember no matter what you use make sure you use antisize compound on all the bolts, so you can get them out if you have to the next time

 

[Alpine 1789]

I used grade 8 on my Tiger and Alpine in the front and where I thought it needed it but remember no matter what you use make sure you use antisize compound on all the bolts, so you can get them out if you have to the next time

Let's see: the current suspension has been there for 48 years. I am 61. So, I shouldn't have to do this again until I am 109. Yep, better use antisize compound!

 

[DanR]

Thanks Bill for the helpful site....

A quote from which is as follows: "Getting back to the original question, “which fastener grade should I use?†We hope it’s very clear by now that grade 8 fasteners are far superior to grade 5 fasteners. If this is so, then why do the automotive manufacturers use some grade 5 fasteners? The automotive OEM’s use what it needs to be safe and nothing more since there is a difference in cost between grade 5 and grade 8 (or metric 8.8 and 10.9). Since the OEM’s manufacture millions of vehicles each year, the difference in a few cents per fastener adds up to a lot for them."

It's a safety vs cost thing! Safety First!

 

[RootesRacer]

Superior is depending on the application.

The difference between grades of hardware usually is the heat treat process. Making a fastener harder has its downside too. The increased hardness makes the part more brittle. Think about the difference between taffy and glass.

For engine parts, grade 8 all the way.
Anything that doesnt get shock loads, grade 8 again.

For suspension components and anything that takes hard shock or has the fastener in a sheer state, I'd go grade 5.

 

[Bill Blue]

Thanks Bill for the helpful site....

A quote from which is as follows: "Getting back to the original question, “which fastener grade should I use?†We hope it’s very clear by now that grade 8 fasteners are far superior to grade 5 fasteners. If this is so, then why do the automotive manufacturers use some grade 5 fasteners? The automotive OEM’s use what it needs to be safe and nothing more since there is a difference in cost between grade 5 and grade 8 (or metric 8.8 and 10.9). Since the OEM’s manufacture millions of vehicles each year, the difference in a few cents per fastener adds up to a lot for them."

It's a safety vs cost thing! Safety First!

Not really. Replacing a bolt that will never fail with a "safer" bolt does not increase the safety factor.

By the way, not counting misuse of a bolt, say reusing torque to yield bolts, when was the last time you had an OEM bolt failure?

Bill

 

[DanR]

Just a thought, Remember when we used to joke about "japanese" manufactured stuff?

Now it's Chinese! But, they are taking over near "all" manufacturing.

Thanks to our government and coprorate functions operating on our tax momies.

I believe the day will come when our desire for 'cheaper' goods will lead to our downfall. Such as "fasteners failing to do what they were designed to do.

 

[Bill Blue]

Keep in mind the cap screws. They have a tensile strength substantially great than Grade 8 and are used in industrial applications that have continuous shock loads never imaged in an automotive application. I thinking we are splitting hairs that do not exist.

Bill

 

[chazza]

A person can hear a lot. Unfortunately, a sizeable portion of it is pure rubbish. The Grade 8 vs. Grade 5 debate is one of them. Here is some real information on the issue.

http://tinelok.com/grade-5-vs-grade-8-fasteners/

/QUOTE]
Not that this article is the gospel - the author advocates torquing with dry threads!

"Real information" I don't think so!

 

[Bill Blue]

Okay, we'll say it is an obvious fact - Grade 5 bolts can work where Grade 8 fail. Now, show some examples where that has proven to be the case. And I'm not talking about your 3rd cousin talking to a guy in the bar whose best buddy crews on a race car and the Grade 8 air filter bolt failed.

Examples that you are aware of.

Bill

 

[jumpinjan]

What about cases where the bolt is going into a captured nut or threaded area on the car, as would be the case with the suspension mounting bolts? Do you worry when a Grade 8 bolt is going into a softer thread?

I don't worry at all, actually feel content. Now, if the original bolts look good, and not rusted or worked on/off a lot, I reuse them. Especially the trans fasteners, clutch bolts (they have a good shoulder like a dowel pin) and some engine bolts. However, I'm now replacing the main bearing bolts with new grade 8.
Jan