When is Grade 8 not Grade 8?

k_enn · Posts: 259 Join date: 2008-07-02

When it is metric.

As some of you know, I have been having trouble with the bolts breaking that hold the Warn manual hub onto the wheel. The original studs that came with it were far too weak (and one even broke before reaching the specified torque). So I went and replaced them with grade 8.8 bolts. Even with the replacement bolts, I kept having a lot of breakage.

Stupid me -- I assumed that grade 8.8 metric was the equivalent to US grade 8. No way. It is actually closer to Grade 2. No wonder the bolts kept breaking. I found out that US grade 8 is metric grade 12.9. Problem is, you cannot find metric grade 12.9 at your local hardware store, or the hardware megastore. I ended up ordering them over the internet.

So yesterday, I spent aver 6 hours extracting four of the six bolts that broke on my passenger side. They broke inside the wheel hub, and it was a real nasty job getting them out. Last time I tried it, I broke three extractors and in the process only removed one bolt. This time I drilled the daylights out of the broken bolts, and eventually got them out with an extractor.

So now, I have replaced all the bolts on both hubs with grade 12.9. I better not snap any more -- these would be super tough to drill and extract.

k_enn

yrflu · Posts: 170 Join date: 2009-07-20 Location: Annapolis, MD

I have been bit by that same stinkin' bug many times. I always try to replace any bolt with the grade 8. But those stupid metrics always suck. And everything on a TJ is metric. bleh. Or stupid torx. Replace those with normal bolts too. That will save you in the long run.

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k_enn · Posts: 259 Join date: 2008-07-02

yrflu wrote:

I have been bit by that same stinkin' bug many times. I always try to replace any bolt with the grade 8. But those stupid metrics always suck. And everything on a TJ is metric. bleh. Or stupid torx. Replace those with normal bolts too. That will save you in the long run.

I never knew that they used a different grading system for metrics. I went to the hardware stores, asked for grade 8, and they directed me to the grade 8.8 metric. Since that was the highest grade they had, I assumed it was the same as US grade 8. Duh. It is a completely different scale that they use on metrics with respect to the bolt grade.

I really don't mind metrics -- the system of sizing is rather simple when it comes to the threads. It is just the bolt diameter and the thread pitch, which is usually one of two possible pitches (e.g., 8mm bolt with either 1.00 pitch or 1.25 pitch). The old US system could be really a mess with all the different possible thread sizes. I have so many old taps and dies and thread measuring gauges that it is crazy.

The one real problem with metrics is availability. You can find the grade 8.8 pretty easily at a decent hardware store or a mega-store. But if you want the higher grade stuff it looks like you pretty much have to order specially from a bolt supplier. That stinks, because you need to know exactly what you need -- you cannot just walk and say "give me one of these."

k_enn

yrflu · Posts: 170 Join date: 2009-07-20 Location: Annapolis, MD

I have 3 fastenals within 15 miles of my house. I think that is the name. They are wholesale supplier for things like that. I have had scary luck going to the small ACE up the road from me. They are expensive, about double my home depot, but they have everything bolt wise it seems.

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JeeperX · Subject: Re: When is Grade 8 not Grade 8? Mon Jul 27, 2009 6:11 pm

Fastenal ROCKS thumbs up

I try to go there whenever I can.

FSJkid90 · Subject: Re: When is Grade 8 not Grade 8? Mon Jul 27, 2009 9:03 pm

Whats so hard about U.S. bolt sizes? Just like the metrics you have the bolt diameter and thread pitch. And there are only two thread pitches per bolt size, National Coarse and National Fine. And U.S. thread pitches are a lot easier to figure out if you don't have a thread pitch gauge handy. U.S. is TPI, or Threads Per Inch. But the silly metrics measure in Millimeters per thread, so you actually have to try to measure the distance between the threads as opposed to just counting them.

Just out of curiosity though, where did you find out the a grade 8 is equivalent to a M12.9? I always thought the grade 8 equivalent was 10.9, and 12.9 was a grade 9. And yes, there is such thing as a grade 9 bolt. And you do NOT want to break one of them off.

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k_enn · Posts: 259 Join date: 2008-07-02

FSJkid90 wrote:

Whats so hard about U.S. bolt sizes? Just like the metrics you have the bolt diameter and thread pitch. And there are only two thread pitches per bolt size, National Coarse and National Fine. And U.S. thread pitches are a lot easier to figure out if you don't have a thread pitch gauge handy. U.S. is TPI, or Threads Per Inch. But the silly metrics measure in Millimeters per thread, so you actually have to try to measure the distance between the threads as opposed to just counting them.

That may be now -- but I have an absolute ton of taps and dies of different pitches, with more than two thread choices for a given diameter. Maybe it was because I got them from my grandfather who was a machinist and tool and die maker, and he needed to fabricate all kinds of stuff.

FSJkid90 wrote:

Just out of curiosity though, where did you find out the a grade 8 is equivalent to a M12.9? I always thought the grade 8 equivalent was 10.9, and 12.9 was a grade 9. And yes, there is such thing as a grade 9 bolt. And you do NOT want to break one of them off.

Actually, I saw it on a website, which I cannot locate now. Upon further investigtion, it looks like you are right -- metric grade 12.9 is a little bit above US grade 8

k_enn · Posts: 259 Join date: 2008-07-02

FSJkid90 wrote:

And yes, there is such thing as a grade 9 bolt. And you do NOT want to break one of them off.

Yes, I broke four of the six bolts. Guess that means I will be replacing the wheel hub assembly rather than drilling them out.

k_enn

tim13con · Subject: Re: When is Grade 8 not Grade 8? Tue Oct 20, 2009 2:14 pm

FSJkid90 wrote:

Just out of curiosity though, where did you find out the a grade 8 is equivalent to a M12.9? I always thought the grade 8 equivalent was 10.9

I agree with Grade 8 = 10.9

UpnOver · Posts: 35 Join date: 2009-01-30

JeeperX wrote:

Fastenal ROCKS thumbs up

I try to go there whenever I can.

Chris next time try A & A bolt on 40. The owner lives near me.. If he doesnt have what you need in stock they will make it for you usually in 24 hours.

rockready · Subject: Re: When is Grade 8 not Grade 8? Tue Oct 20, 2009 7:50 pm

never thought of it this way until my cousin brought it up. grade 8 and stuff high like that has a easy snap point meaning if you bend it it snaps quick but a grade 5 will bend and not snap ( side to side pull ) as fast. seems your snaping the bolts due to to hard of bolts. grade 8 is good if your trying to pull it apart. not sure if i explained it right. not trying to step on toes. just giving my 2 cents.

tim13con · Subject: Response borrwed from rockcrawler.com Wed Oct 21, 2009 10:13 am

rockready wrote:

never thought of it this way until my cousin brought it up. grade 8 and stuff high like that has a easy snap point meaning if you bend it it snaps quick but a grade 5 will bend and not snap ( side to side pull ) as fast. seems your snaping the bolts due to to hard of bolts. grade 8 is good if your trying to pull it apart. not sure if i explained it right. not trying to step on toes. just giving my 2 cents.

I’ve also heard the argument that grade 8’s are more brittle than grade 5’s and that’s why you shouldn’t use them. Well, first you need to understand what the term “brittle” really means. Brittleness in bolts is defined as failure at stresses apparently below the strength of the bolt material with little or no evidence of plastic deformation. Typically, fasteners are not brittle below 180 ksi ultimate tensile strength. Grade 5’s have an ultimate tensile strength of 120 ksi and a grade 8 fastener has an ultimate tensile strength of 150 ksi. This is why brittle is a relative term. Nearly all fasteners are considered ductile except some made from PH 15-6 Mo, 17-4 PH and 17-7 PH.

Going back to the D-ring on the face of the bumper example, you would want to know its tensile carrying capability. Calculating the tensile capability is not as easy as shear since the thinnest portion of the bolt is at the minor diameter of the threads (bottom of the thread “V”). So you need to know the nominal minor diameter of that particular fastener. That’s where military specification MIL-S-8879C comes in. It is titled “Screw threads, controlled radius root with increased minor diameter, general specification for”. It lists that and a lot more for almost all possible fasteners. MIL-S-8879C lists the nominal minor diameter of a .2500-28-UNF at .2065 inches. We can now calculate the A (area) of the cross-section:

A = Pi x r2 = Pi x (.2065/2)2 = .03349 in2

Grade 8 bolt capability in yield (stretch) = 130,000 lbs / in2 x .03349 in2 = 4354 lbs minimum

Grade 8 bolt capability in tension (failure) = 150,000 lbs / in2 x .03349 in2 = 5024 lbs minimum

Grade 5 bolt capability in yield (stretch) = 92,000 lbs / in2 x .03349 in2 = 3081 lbs minimum
Grade 5 bolt capability in tension (failure) = 120,000 lbs / in2 x .03349 in2 = 4019 lbs minimum

Again, you can see that the grade 8 will support over 1000 lbs more or a 1/2-ton more. But there’s something more important to note. The grade 5 fastener has already reached its ultimate load and FAILED BEFORE the grade 8 starts to yield or stretch. Therefore, the argument that you should not use grade 8’s because they are more brittle than grade 5’s is not a true statement in most applications.

rockready · Subject: Re: When is Grade 8 not Grade 8? Wed Oct 21, 2009 4:35 pm

well killed that thought. lol!

JeeperX · Subject: Re: When is Grade 8 not Grade 8? Wed Oct 21, 2009 10:24 pm

Wow can you put that in English please? lol!

AutoDoc · Subject: Re: When is Grade 8 not Grade 8? Thu Oct 22, 2009 7:38 am

Looks like somebody's been going to engineering school!

Here are the redneck translations:

#1: 8 is more than 5 so it's better.

#2: Grade 8 costs more than grade 5, so it's better.

AutoDoc's rule of thumb: If you broke a grade 5, replace it with an 8. If the 8 breaks, get out the welding machine.