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Grades of Bolt Strength

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kthompson
Join Date: Oct 2005
Posts: 5219 South Carolina
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2008-04-03          152692


Many do not know they are different grade bolts (also nuts)and some have no idea how to tell the difference if they know. For standard bolts there are dashes on the head of the bolt to identify the grade. The common grade (2 or 3) has no dashes, the other grades has them and you ADD two grades to the number. So a grade 5 has 3 dashes and grade 8 has 6. (Metric is different.) If you are using them for the shear load the nut may not be that important but if you are using the for a load pulling agains the threads but sure you use at least as high as grade nut.

Probably not always but often you can look at the bolts very closely and tell the metal in the common grade bolt does not look as fine a grain as the higher grade bolts does. Not sure you can depend on it but my memory is all the common or grade two bolts I have seen are silver in color while often the grade 5 and 8 are not. But that is why the head it mark, to remove the guess work.

BTW, the troque value is much higher on the higher grade bolts. So is the shear load. The info below I copied for one of many web sites where you can get more details. kt

Grade - 2

Low Carbon Steel. Economical for use in non-critical applications where the fastener is not subject to extreme temperatures or stress.

Grade - 5

Medium Carbon Steel, Quenched and Tempered. Has greater tensile strength than a Grade-2. Yield-to-tensile ration is the lowest of all heat-treated steels.

Grade - 8

Medium Carbon Alloy Steel, Quenched and Tempered. Has greater tensile strength than a Grade-5. Designed for use in high stress applications.



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Grades of Bolt Strength

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kwschumm
Join Date: Feb 2003
Posts: 5764 NW Oregon
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2008-04-03          152697


Why don't the manufacturers just STAMP A NUMBER on the bolt head? This is the kind of information you can read a dozen times and not remember when needed because it's so obscure. ....

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Grades of Bolt Strength

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Murf
Join Date: Dec 1999
Posts: 7141 Toronto Area, Ontario, Canada
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2008-04-03          152700


That would be too easy.....

Besides, just like the whole imperial / metric debate, nobody wants to change their system.

In Metric, or more properly ISO, there is a numeric identifier on the head of a bolt. It is usually either one or digits followed by a decimal and another digit, this gives a much broader range of strengths and descriptions than the imperial ones do.

Best of luck. ....

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Grades of Bolt Strength

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lencamp
Join Date: Oct 2006
Posts: 29 Trufant, Michigan
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2011-01-08          176285


Don't forget L9 and how about stainless steel. Just heard recently that SS was a poor conductor of electricity - didn't know that. Grade marking info is common - see it in every service manual. ....

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Grades of Bolt Strength

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auerbach
Join Date: Sep 2007
Posts: 2168 West of Toronto
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2011-01-09          176316


KW, to understand the reason makers can't emboss more than the simplest of specs on the boltheads, if Ken provided the full picture, we'd still be reading it.

For instance, we talk of "stainless" (which is usually above Grade 2 in common measures of strength, and typically low conductivity and non magnetic) but there's any number of non-corrosive formulae. Moreover, strength is measured in several ways, and sometimes (as in head bolts) what's equally important is heat resistance and elasticity. It would take a book to fully cover the metal itself, and that's just part of the picture.

For instance, there are different configurations of head to shaft. Threads can be fine or coarse, deep-cut or shallow. Nuts can be different dimensions (apart from the thread size) and various materials. There are four standards for how tightly the nut grips the threads. With high specs (typically high grade, fine thread, deep cut, max tightness) you next address washer specs because a common washer would get squeezed to nothing.

If you're writing the specs for, say, engine-mount fasteners, it matters whether it's for a car, ship, or aircraft and you can't stamp all that info on the bolthead. And that's just for what's on the shelf; in some applications the fasteners are custom.

For servicing tractors, all we need is what Ken provided. ....

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Grades of Bolt Strength

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skipll
Join Date: Feb 2011
Posts: 184 Robbisville NC USA
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2011-09-25          180559


Just saw this----very interesting---I never knew what the marks ment on bolts--though I knew there were diff. grades. Just went out to my shop as I keep a good supply of new bolts of diff. sizes-----had a peek---95% are grade 5---but thats mostly all I need. ( few grade 8 too)

Thanks KT for posting & all others that added info. ;) ....

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Grades of Bolt Strength

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skipll
Join Date: Feb 2011
Posts: 184 Robbisville NC USA
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2011-09-25          180560


Quote:
Originally Posted by Murf | view 152700
That would be too easy.....Besides, just like the whole imperial / metric debate, nobody wants to change their system.In Metric, or more properly ISO, there is a numeric identifier on the head of a bolt. It is usually either one or digits followed by a decimal and another digit, this gives a much broader range of strengths and descriptions than the imperial ones do.Best of luck.

Hey Murf---I got some metric bolts stamped 8.8----what would be the strenght level of that? ....

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Grades of Bolt Strength

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Murf
Join Date: Dec 1999
Posts: 7141 Toronto Area, Ontario, Canada
TractorPoint Premium Member -- 5 Tractors = Very Frequent Poster  View my Photos  Pics

2011-09-25          180562


Skip,

There's no direct conversion, but a grade 8.8 Metric bolt is about as close to an Imperial grade 5 as 'dang it' is to cussing. :)


Best of luck. ....

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