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Unread 10-22-2008, 07:23 AM   #1
ksm4_al
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Grade 5 vs. Grade 8 Fasteners

Although I originally posted this response to a question on a different thread the article on ROCKCRAWLER.com by David Potter is so well written I thought the post was worth repeating here.

Quote:
Originally Posted by 4.7stroker View Post
OK Why does everyone insist on grade 8 hardware?
Grade 5 is just fine and much cheaper, the better safe than sorry answer will not cut it with me.

Dwayne
Hey Dwayne. David Potter's article from ROCKCRAWLER.com will will do a much better job answering your question than I ever could seeing that he is a "genuine rocket engineer for Lockheed Martin".


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Unread 10-22-2008, 07:35 AM   #2
Unlimited04
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Here's my personal experience with grade nothing vs grade 5 vs grade 8 & 35 ft-lbs of torque.

http://www.jeepforum.com/forum/f27/why-use-grade-8-hardware-606505/

If you didn't understand all the shear & tension engineering speak in the above link - just buy grade 8 and don't ask questions
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Unread 10-22-2008, 12:50 PM   #3
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its all about the tensions and shear strength, I personally always like to overkill the things I do, like my roof rack is bolted down with M10.9 bolts which are equal to Grade 8 bolts, and I have even broke a M10.9 bolt in half on my Jeep, it was a bolt that held on the engine skid and I hit it real hard once and the head just snapped off
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Unread 10-22-2008, 01:08 PM   #4
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Quote:
Originally Posted by jsc7002 View Post
I have even broke a M10.9 bolt in half on my Jeep, it was a bolt that held on the engine skid and I hit it real hard once and the head just snapped off
In many areas, like the case above, a grade 5 will work better. Being the grade 8 is much harder than a grage 5 bolt it is also more brittle. You are not going to bend a grade 8 bolt easy, but they can crack and break when the reach a certain point. On something like an engine skid that has a certain amount of give to it a grade 5 bolt is a better choice.
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Unread 10-22-2008, 04:33 PM   #5
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A Grade 8 will do everything a Grade 5 will do... except better... and at a higher cost.
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Unread 10-27-2008, 11:45 AM   #6
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Quote:
Originally Posted by Bigbob View Post
In many areas, like the case above, a grade 5 will work better. Being the grade 8 is much harder than a grage 5 bolt it is also more brittle. You are not going to bend a grade 8 bolt easy, but they can crack and break when the reach a certain point. On something like an engine skid that has a certain amount of give to it a grade 5 bolt is a better choice.
A Grade 5 bolt will ALWAYS ALWAYS ALWAYS fail before a Grade 8 bolt will, no matter what use or stress it is subjected to.

A Grade 8 bolt is not so hard that it would ever fail before a Grade 5 bolt would, no matter how you flex/stress it. There is no use that a Grade 5 would hold up to better than a Grade 8 bolt would. That said, I use Grade 5 bolts when Grade 8 is not called for. I don't use Grade 8 bolts for everything as some people seem to think is necessary.
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Unread 10-28-2008, 09:23 PM   #7
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Quote:
Originally Posted by jsc7002 View Post
its all about the tensions and shear strength, I personally always like to overkill the things I do, like my roof rack is bolted down with M10.9 bolts which are equal to Grade 8 bolts, and I have even broke a M10.9 bolt in half on my Jeep, it was a bolt that held on the engine skid and I hit it real hard once and the head just snapped off
i have actually stretched an M10.9 bolt on my UCA with a 1/2" drive socket and box wrench to the point that when i went to torque it to the stock 55ft/lbs, it merely ripped in half. so i went with true grade 8. maybe it was a bad bolt but i didnt want to risk it.
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Unread 11-02-2008, 05:21 PM   #8
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Several years ago I simply just started refilling the bolt bin with Grade 8 hardware and I have started adding extra bins with stainless hardware where there isn't a needfor the strength but corrosion may be a factor.
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Unread 11-02-2008, 05:30 PM   #9
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Using a grade 8 fastener is only "effective" once you apply a torque (read clamping load) close to the yield point of the fastener. Because of the Heat treated condition it will require more load to stretch, hence it will generate more clamping load..A grade 8 bolt used with low clamping loads will behave worse then a grade 5.. Also keep in mind that the nut or threaded hole the fastener threads into has its limitations vis a vis yield....
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Unread 11-02-2008, 06:22 PM   #10
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The clamping load might explain something that happened to me a few years ago. I replaced all my spring bushings and installed new grade 8 bolts in all the spring eyes and shackle hangers. Shortly after I was on a very difficult trail when I heard a banging noise. I had broken one of the new grade 8 bolts on the front of the rear spring. I replaced the bolt and went on my way. When I got home, thinking about what had happened, I replaced all the bolts with grade 5 and never had another problem. I believed that the impact from the spring on the hardened bolt, which is more brittle, caused the problem. I used grade 5 because I believed a slightly softer bolt would bend before breaking. Now, I may have just had a defective bolt. I'm not sure, but the clamping force was not that great to allow the spring to move. This might explain things. I have used grade 8 since for my winch and roll bar. I wouldn't think of using any less for those items.
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Unread 11-03-2008, 08:58 AM   #11
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Quote:
Originally Posted by USAntigoon View Post
Because of the Heat treated condition it will require more load to stretch, hence it will generate more clamping load..
Not a true statement - the stiffness doesn't change with heat treat. Steel's modulus is ~30ksi regardless. A gr8 bolt isn't any stiffer than a gr5 bolt, until you exceed the proof load of the gr5 and lose some of the preload.
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Unread 11-03-2008, 10:13 AM   #12
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I used the word "stretch" and I was thinking "stretch to proof load", because this is way higher for grade 8 bolts then grade 5. (sorry)
Those who are now even more confused should read THIS article and this one as well
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Unread 11-03-2008, 10:22 AM   #13
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I read a book from Barnes & Noble called "Fasteners and Plumbing" or something like that.. it had more info about nuts/bolts than I needed to know but it was pretty interesting. My wife couldn't believe I would lay in bed and read about "stupid bolts" as she called them.
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Unread 11-09-2008, 10:24 AM   #14
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^ A true sign of a fabricator! nice Besrk!
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