Grade 5 bolts versus Grade 8 bolts

[w squared]

In passing, I seem to hear grade 8 bolts spoken of as the standard when it comes to bolting recovery-related items onto your jeep. The manual for the Warn 9.5ti that I now own (thanks Wheels!) specifies the following hardware:

3/8-16 X 1 1/4" long, grade 5 bolt
torque 30-35 ft. lbs

Length/diameter is simple enough. It needs to fit.

Likewise torque. It needs to be tight, but if you hand a breaker bar to a gorilla, he'll destroy your hardware before you ever hit the trail.

My question is this: What's the difference between the grades of bolt? A quick look at Wiki (NOT my preffered source for this type of information) suggests to me that a grade 8 bolt is going to be stronger. Is this the case? If so, should I be concerned about the harder steel being brittle? I don't mind spending a few extra dollars to get the best hardware commonly available. I know what can happen when hardware fails.

Or should I just trust that the engineers at Warn that wrote the spec have their heads attached properly and put in the grade 5 bolts that the manual calls for?

[frenchy]

Grade 5 will bend and then snap before a grade 8 starts to bend.

[Mudpig97]

A grade 5  has a greater resistance to shearing than a grade 8.  A straight pull on a winch will try to shear the bolts off.

[frenchy]

A grade 5  has a greater resistance to shearing than a grade 8.  A straight pull on a winch will try to shear the bolts off.

Isn't the shear strength just 60% of the tension strength?
Since 8 has higher tensile strength, it's shear strength should also be higher.

[dunl]

A good read:

http://www.rockcrawler.com/techreports/fasteners/index.asp

[BlackYJ]

Grade 8 fasteners are a must in my opinion for any recovery hardware, ie tow hooks, for safety.  Now if you look at the shear capability of a 3/8" grade 5 bolt for you winch, 5750 lbs each and there are four bolt holding the winch.  The winch will never see more than 9500 lbs at the winch which is less than the capability of 2 bolts

I like to use grade 8 hardware as much as possible because of the zinc coating as it helps prevent the bolts from seizing

[w squared]

A good read:

http://www.rockcrawler.com/techreports/fasteners/index.asp

Fantastic! Bookmarked.

I know enough about metallurgy and physics to nod my head along with the author of that article. He does a great job of explaining the differences and justifying his choice of Grade 8 hardware.

I was sold even before I considered the zinc coating 

Thanks guys.

[JohnB]

Other than the strength increase, grade 8 bolts are generally built to a higher quality.  Grade 5 fasteners can vary a lot in quality.

If you want really good stuff, get ASTM A193 B7....   Mmmmmmm....   But anyway, plated grade 8 should always be your normal choice in a stressed component.

[bae146]

If you know any aircraft mechanics with access to aviation hardware they can get you really nice hardware that exceeds any grade 8 spec.  And the pricing is not bad either.  The good stuff comes with a radius under the head of the bolt with a special countersunk washer to go underneath.  Locking nuts on the other end as well will prevent any loosening.

[hps4evr]

my dad used t owork in a tractor manufacturing factory. he brought home some bolts for me to use when building an old bumper i had. they werent huge, but he said they were grade 12 metric bolts. you usually see an 8.8 or 10.9 metric bolt. i thought it was cool. over kill. he now works for boeing, the airplane company. he brought home some titanium bolts for me to look at. very strong, and very light. but he had to return them:(
if everyone suggests using grade 8 over grade 5, why do warn winches come with grade 5 hardware? ive been told that grade 5 will bend in a shearing situation where a grade 8 will break in that same situation. a grade 8 has a stronger clamping ability. if you're mounting front facing bolts the grade 8 would be the ticket. but i think a grade 5 will work for the other bolts. i use a mix of grade 5 and grade 8 and some metric 8.8 and 10.9 bolts for my current winch and bumper combo. but i think in the end as long as its bolted down properly then it will hold. but the zinc plating of the grade 8 bolts is a nicer touch. also look into fine threads over course threads. either one may work better in your situation.

[cuzican]

As the winch is behind the winch plate there shouldn't be huge stress on the bolts holding the winch to the winch plate. The winch plate takes most of the load.

However, I'd be concerned if Warn suggested grade 5 bolts to secure the winch plate to the chassis.

[Trouble Maker]

Gr 5 bolts are generally Zinc plated, silver in color.

Gr 8 are Cadmium plated, gold in color.

[cLAY]

Grade 5 bolts will have 3 lines on the head, grade 8 will have 5 or 6. A gr8 will usually snap before it bends(more brittle) but a gr5 will often stretch or break.  An example given to me in tech school was that Mack trucks had a problem with snapping the fly wheel bolts. Turned out gr8 was too brittle for the application(vibration) so they changed out to gr5 and all was good.

The  reason why Warn says to use Grade 5 is simply because thats all thats needed. As said above

"if you look at the shear capability of a 3/8" grade 5 bolt for you winch, 5750 lbs each and there are four bolt holding the winch.  The winch will never see more than 9500 lbs at the winch which is less than the capability of 2 bolts"

Its the same reason why an 8000# winch comes with 5/16s cable instead of 3/8s, its because thats all you need. If you want to go bigger or better, go ahead, no harm in that.

An easy supply of super tough bolts is CAT bolts from Finning, not sure what they are rated at but they are better than GR8 and have a beefier head on them for better grip with your tools. I used a set of them when I plated the frame around the steering box on my XJ and needed a longer bolt. Not many people realize those bolts are better than GR8 quality from the manufacture, same with the axle end trac bar bolt, its a 10.9 if I remember right, standard off the shelf bolts are 8.8.