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Old 03-16-2002, 10:32 PM   #1 (permalink)
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Grade 8 vs Grade 5

Here's an exserp from another BB. I wanted to argue that grade 8 is stronger than grade 5 under ANY circumstances but I don't have any specs to back it up. Isn't the "grade 8 will shear when a grade 5 will bend" a misconception? I need specs....

Another thing to remember... you aren't looking for total ballstothewalls clamping force in a beadlock. The higher the grade the bolt the higher the tensile strength... meaning you can increase the torque and clamping force without breaking it as easy... but as tensile strength increases the shear strength usually decreases. A true Grade 8 fastener is more likely to shear than a Grade 5 fastener. The Grade 5 will bend before it breaks, the Grade 8 usually just shears. It's the same thing with axleshafts and gears... a softer gear and axle shaft doesn't make a weaker axle shaft or gear. Drag racing gears are softer to absorb the shock loads at launch... normal street gears would shatter under the load. While using a Grade 8 bolt is usually an ok thing to do, there are applications when it isn't appropriate.
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Old 03-16-2002, 11:07 PM   #2 (permalink)
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The grade 8 will have a higher tensile strength but it's more brittle and will snap before it bends. I had an alternator bracket that I went through mounting bolts quite frequently with. Apparently engine vibration put too much stress on them and they snapped. I had much better luck with grade 2 bolts than I did with grade 8. The lower grade bolts were able to flex with the vibrations whereas the grade 8 were not. This is the exception to the rule though, in most instances a grade 8 will be superior to a lower grade.
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Old 03-17-2002, 01:06 AM   #3 (permalink)
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I've got a strength of materials book in my car (isn't here right now or else I would get some numbers) but IIRC, a grade 5 will flex and then snap before the grade 8 will being to flex at all.

Every material has an elastic limit (a bolt wouldn't be much good without one) but if sheer load capacity is going to be less for a lower grade fastener, why use it? Yes a grade 8 is going to snap before it would distort a great deal, unlike a lower grade. But I wouldn't exactly call it brittle. Maybe I am totally off here though. I'll dig up numbers tomorrow.

Something to consider regarding the gears for dragsters. Yes, they are softer to allow some give at launch, however they also don't last very long at all. Given a few hard launches, the units are fawked and need to be replaced. Something to consider.
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Old 03-17-2002, 05:03 AM   #4 (permalink)
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That excerpt is totally and completely wrong. As tensile strength increases shear strength DOES NOT decrease, in fact shear strength is a FACTOR of tensile strength, and in the case of SAE fasteners is usually in the region of 60%.

My reference is "High Performance Hardware: by Forbes Aid (HP Books) and the Machinery's handbook, 24 edition.

Here is some more info on th subject, posted a couple years ago, by knowlegable folks.

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Grade 8 vs Grade 5 bolts
Maybe I can clarify (or complicate?) things further.
In tension or shear, grade 8 bolts are stronger than grade 5. Period.
For a given load, any steel bolt (grade 8 and grade 5 or grade anything) will stretch the same amount, up to its yield strength (also known as its elastic limit). Remove the load, and it "unstretches" back to its original length, i.e. there will be no permanent stretch (like a spring). For a grade 8 bolt, this load (and corresponding stretch) is greater than a lower grade.

Above the yield strength, the stretch increases dramatically with very small increases in load, until the ultimate load is reached. Again, the ultimate load is higher for a grade 8 bolt than it is for a grade 5. Furthermore, if you stretch a bolt beyond its yield limit, it will not return to its original length (like a spring you've stretched out of shape).

Softer steels will stretch more than harder ones before they break, but the actual load at the break point will still be much lower for the soft steel.

I used tension examples, but shear behaves in much the same way.

Then there's fatigue strength. Bend a piece of metal back and forth a bunch of times; it may not break right at first, but it will break eventually. For any given metal, there is a fatigue limit, which is lower than the ultimate load. Below the fatigue limit, bend it as many times as you want and it won't break; above that, it will eventually break. Again, in general, the higher the ultimate strength, the higher the fatigue limit.

Finally there's impact loading. This is tested by notching a steel bar and hitting it with a hammer (very precisely, of course). This is the one case where a softer steel might do better, as harder materials tend to be more brittle. However, for most of our applications it's not that important (unless you're running a snowplow or trying to snatch a stuck Jeep out with a chain instead of an elastic nylon strap), as the suspension tends to damp out and spread loads below the sharp spike necessary for impact failure. Also, even grade 8 bolts aren't all that brittle; after they're heat treated to get the ultimate strength up they're tempered to bring back the impact strength. Impact strength might be an issue if something's loose, so that it shifts and bangs around before it hits whatever is stopping it (i.e. the loose bolt).

Conclusion: grade 8 bolts are stronger in virtually any application on a Jeep. Whether you need that strength, and want to pay extra for it, is another issue (though I bought some 5/16 grade 8's yesterday for $0.60 each; grade 5's were only ten cents cheaper). There may also be situations where you may want the bolt to fail before the expensive part it's holding breaks, but that's another issue.

FWIW, "aircraft grade" bolts (AN or MS) are generally grade 8's will additional testing and quality control requirements-- not a bad idea for the bolts holding your wing on at 10,000 feet.

Fine or coarse thread? Fine thread bolts are generally stronger in tension as they have a greater cross-sectional area (the finer threads don't cut as deep). They can also develop greater clamping forces at reasonable torque values (cylinder head bolts are a good example). However, when threaded into a softer metal (like aluminum) coarse threads have less of a tendency to strip out, and are the better choice.

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posted 01-22-2001 10:40 AM
Austnjpr is right. I sell fasteners for a living. If you want to be extra cautious you can use some of the above grade 8 "bolts", such as Bowmalloy, supertanium,or Karalloy, etc. Nascar builders use Bowmalloy, though there is only 2 actual manufacturers of U.S.A. hex head caps screws (bolts) The above grade 8's have the ductility (ability to bend) of grade 5, and can create a greater clamping force than a grade 8.
One thing to keep in mind when discussing shear is slip factor. the slip factor is the total amount of force required to overcome both the natural friction of the surfaces of the bolted material and the fastener used in the aplication. Or the force required to cause the joint planes to slide and contact the shank of the bolt.(not a good thing)

The way to increase slip factor is to increase clamping force. for example the single shear strengths for a 5/8" coarse bolt are
grade 2 = 11,200 lbs.
grade 5 = 18,170 lbs.
grade 8 = 22,700 lbs.
Bowmalloy = 27,250 lbs.

The slip for properly torqued bolts are as follows this is on a mill scale finshed surface. (too lengthy to dicuss here, but you should get the idea)

grade 2 = 3,270 lbs. @ 97 ft. lbs.
grade 5 = 5,040 lbs. @ 150 ft. lbs.
grade 8 = 7,100 lbs. @ 228 ft. lbs.
Bowmalloy = 8,600 lbs. @ 290 ft.lbs

so the fastener capable of being torqued to a higher value will be stronger in shear. The most common mistake resulting in fastener failure is improper loading of the fastener. the best way is to measure stretch. we can't always do this so we often use the second best method which is torque values. I have used grade 8 USA made fasteners (or better) in my rig properly torqued and they do not loosen.

ASTM grade A325 is the equivelent to a SAE grade 5 in terms of tensile strength they are exactly the same. the difference is the construction fastener has different shoulder verse thread length. H8 monday if you have replaced any factory grade 8 bolts these would be a downgrade. otherwise you have grade 5. Watch out for ASTM grade A307A these are the equivelent to a grade 1 or2 they are sold at home depot. They have no business being on your rig they may kill someone if used for suspension.

Another thing to watch out for is "suspect bolts" these are made in taiwan or other foriegn countries. Though they may be made to standards how do you know? An example again is some of the head marking they should have an n on them Newcore sp? or a triangle for Lake Eerie. The other possibility is the legit distributers such as Bowman, Kar, Lawson, Premier, and a few others use bolts for one of these 2 and have their own logo put on.
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Old 03-17-2002, 07:44 AM   #5 (permalink)
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Old 03-17-2002, 07:44 AM   #6 (permalink)
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Old 03-17-2002, 08:32 AM   #7 (permalink)
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Thanks Billa Vista.

I know this has been beaten to death, but I can't stand it when someone starts spouting ignorance. Someone who reads it and doesn't know any better will think it's better to use grade 5, in a bumper application for example. It's dangerous. I'll post some of these specs and hopefully educate some people.
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Old 01-23-2014, 11:24 PM   #8 (permalink)
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Thanks Billa Vista.

I know this has been beaten to death, but I can't stand it when someone starts spouting ignorance. Someone who reads it and doesn't know any better will think it's better to use grade 5, in a bumper application for example. It's dangerous. I'll post some of these specs and hopefully educate some people.
It's getting so tiring to read people saying "I'm using a grade 5 bolt for my brake caliper because it will bend before it breaks! What stupid engineers must have designed it and used grade 8 bolts which have a lower shear strength than grade 5 but higher tensile strength"

This is simply just idiocy! Shear is directly related to tensile strength in essentially all applications that we as automotive users will encounter. The old theory that the grade 8 bolt is more brittle is also shite.....maybe there is more hardness to it, which would, in theory, make it more brittle, but it is relative to the strength of the bolt to begin with so it is still far stronger in every way than a grade 5!

Oh and for guys who really want ultimate strength....go metric! 12.9 metric bolts have a tensile strength rating of between 170,000 and 180,000 psi whereas grade 8 tops out at 150,000 from what I recall. Either way, they both strong, but the 12.9 is where it's at without going to an arp or srs (think that was the abbreviation) or a super alloy metal, such will cost you your children's college fun as well as your anal virginity.....
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Old 01-23-2014, 11:40 PM   #9 (permalink)
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way to bring up a 12 year old thread and say nothing
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Old 01-24-2014, 12:24 AM   #10 (permalink)
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