Grade 5 or 8 bolts for hitch?

And what about metric for the factory nut strips, what grade metric should I use.

8 if I recall correctly. -Shawn

Just spent a good part of the afternoon reading up on bolts, grades and strength considerations. I am by no means an expert in this area, but I think I would go with grade 8 for the hitch and for the metric nut strips at least an 8.8, but better yet with a 10.9 if possible. Check this out.
Brent.

Thanks guys. Good info on metric bolts.

grade 8 at least. higher tensile means a tougher bolt in most cases. an 8 will break where something like a 5 would tend to stretch rather than just break...but a 5 has a lower tensile. tensile is the force applied to a material by pulling it apart (other forces include; shear, compression, torque and impact...)
kinda like bubble gum a lower grade tends to be more plastic, but if you were to "treat" a lower grade to make it stronger, and in this case harder, it would be akin to putting the gum in the freezer making it harder to chew.
do some research into who made the bolts and the process, because in my limited experience not all 8's are created equally. IMHO

Hmmm, got into this discussion before, Unless I'm mistaken the winch plates are SHIPPED with grade 5 from the manufacturers and the reason is that the grade 8 will fail all of a sudden, with grade 5 you get some warning when they stretch instead of watching the winch head downrange when the grade 8's let go. I also seem to remember that while grade 8's work great for head bolts where the tension is straight up against the bolts they are not quite so strong when the tension is 90deg off or at a right angle. The army used grade 8's on the armour on our bradleys from the factory, when the M2's dropped or were airborn the side armour would drop off from broken bolts when you came down, they replaced them with grade 5's, same on the M1's, BUT, that was also during the same time period when they got a whole lotta bogus grade 8's in the system too when the contractor gun decked the destructive testing results to save money.
The 201 jeep course that I went to this past summer also recommended grade 5 on the winch plate for that same reason, a grade 5 when it stretches and relaxes is still a grade 5, when a grade 8 is overstressed it is not even a grade 5 after that....
I also seem to remember that you don't reuse grade 8 head bolts once they have been torqued and used for any length of time....
Just my .02, I'd ask the warn people myself and see what they say...

thank you for that answer. i saw two answers that i would consider wrong and then that last one (grade 5s) was what i was thinking myself. think of the logic behind it. grade 8s are harder and therefore more brittle. stressed enough they will snap, grade 5s will too but it will be less likely. think of a glass cup and a hard plastic cup (acrylic). you could stand on either probably without breaking it (compression, similar to tension but oppisite direction). if you were to lay a cup on its side, it would break if it was glass and you stood on it. if you were to stand on they acrylic cup it would bend quite a bit and then break. so at the limit there is a little more room for error with the more flexible material.

yea, but you can take that analogy a bit futher, *maybe* that glass cup would not break untill you picked up an extra 100lb weight and then stood on it. Personally I'd rather have some warning other than a quick -groan snap-, the 5's would provide more warning kind of like a tattletale on a heavy line. If the total winch strength of say a 9000lb winch is well within the safety margins of the grade 5's and the grade 8's provide twice that strength then the grade 8's are a waste of money and grade 8's are a alot more expensive than grade 5's. I just had to buy new head bolts for a 89 4.2 in a YJ and that was a shocker, never new those puppies were that expensive. I also thought it was interesting that the length of the old head bolts and the new ones had a visible difference, the old ones were almost a 32" longer, don't know if it was just that they were made that way, different lot variations or if they had stretched out.
The other downside is that if you break a grade 8 they are a SOB to drill out, the 5's are not that bad and here in Pa bolts rust in place in about 3 years even with anti-seize though I now use the copper kind, lasts longer than the silver.
I have also started using again a thomas and betts product called SC40, it's a spray on rust inhibator that coats the threads and puts a seal between the nut and the thread and needs to be removed with a spray cleaner that breaks it down, stuff is marine rated and an anti-corrosive coating, had to special order it thru a local electrical supply house.

Personally, grade 5 not 8. 8's have higher tensile but lower shear than 5's. On a hitch you want the shear strength. PLus gr8 does seem to fail all at once and gr5 gives you warning.

Sarge

If a grade 5 bolt supposedly has a higher resistance to shear when compared to a grade 8 then why is the shear rating of a bolt generally rated as 60% of the bolts tensile strengh in pure axial tension?????

Because it isn't true.

A grade 8 bolt is stronger than a grade 5 bolt IN EVERY WAY!!

The difference in plastice yielding between the two is negligible.

Also, all things being equal a grade 5 bolt will yield and ultimately fail before a grade 8 bolt of the same size used in the same application will even beging to yield.

What matters is not the bolt strength you decide to use for a given application but rather that the chosen bolt strength and size surpasses the applied loads with an adequate safety factor.

In the case of a winch fastener which are generally grade 5 5/16" bolts, 4 of them in shear with a safety factor of 1.5 gives a total load of 15300 lbs. That is well above the strength of a 9k winch cable. For the home mechanic or fabricator the extra $$ to go to a higher grade fastener is nothing...in a production atmosphere those pennies add up so they use the cheapest fatener possible that will resist the applied loads.

So the important point is this.....
If you are going to build something custom make sure the bolt you choose resists the loads with a good sized safety factor. If this is the case yielding will never come into play.

If you are replacing hardware on your vehicle use fasteners equal to the original grade or greater.

I have to agree with Bender, although I don't defend my knowledge quite so vehemently.
I read several times over yesterday while researching bolts and strength that the "grade 8 embrittlement argument" is specious.
Try reading this page: http://www.rockcrawler.com/techreports/fasteners/index.asp
This one's good too: http://www.engineersedge.com/thread_strength/thread_bolt_stress.htm
Beej.

vehemently?? Holy crap Time for me to hit the dictionary!

Uh oh. I opened a can of worms with this one. Maybe I'll just use three grade 8 and two grade 5. What do you guys use in your hitches?

jeepguy97 said:

Uh oh. I opened a can of worms with this one. Maybe I'll just use three grade 8 and two grade 5. What do you guys use in your hitches?

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Ya, this one gets alot of people going, not as bad as the dino vs syn though. As for the hitch. as I mentioned above grade 8's are a sob to drill out if you break one, grade 5's are not, that area rusts out back there pretty bad. Here in Pa it took about 3 years for my skid plate bolts into the frame rail plates to get pretty bad and that was with liberal applications of anti-seize too.. I'd use the same grade bolts that come with the hitch...

jeepguy97 said:

Uh oh. I opened a can of worms with this one. Maybe I'll just use three grade 8 and two grade 5. What do you guys use in your hitches?

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Funny but in mil spec applications before around 1980, that´s exactly what they often did. They used a three hole tow hook, they´d use grade 8 (2) and a larger shank (size) grade 5 in the rear of the hook mount, set in a slotted hole. If the 8´s snapped, the hook would slide in the slot and the rather large 5, would catch the stress. The grade five in the rear, would also help absorb some of the shear in off center pulls. Don´t know why they discontinued the design, either made it cheaper or decided it wasn´t needed.

i agree...mostly. the only reason i said i would go with the 8 is because i enjoy overkill.
i had a chance to test various metals (with standardized test specimens) with a tensile machine. i did notice the (approximate) grade 5 material elongate much more than the grade 8 (again approximate) materials. my favorite was the titanium test...but that is off topic. so IS grade 5 strong enough? well yes and no, size matters, but like beej said 8 is stronger in every way. in testing though i noticed shock loads on the 8 material caused it to crack and fail without warning...shock loads are like what happens to a vehicle dropped from an airplane...sudden impact (like rich p example).
personally from reading this forum, i trust rich's experience and the websites beej posted are informative... just remember quality is important when you come to final solution. the reason i stress quality is a plated 8 will be prone to hydrogen embrittlement (grabbing moisture while hot...basically). a grade 8 is generally the highest grade not inspected...higher grades are inspected for any flaws individually. flaws are stress risers which would cause any material to fail before it was designed to fail. do a search on grade 8 vs grade 5 on google and you will have a wealth of info from metalurgists, engineers and rocket scientists (usually good at math!).
my last 2%

Both my Warn winch plate and my trailer hitch shipped with grade 5, I've put both through their paces for a while now and never had any problems.

PapaPump said:

think of a glass cup and a hard plastic cup (acrylic). you could stand on either probably without breaking it (compression, similar to tension but oppisite direction). if you were to lay a cup on its side, it would break if it was glass and you stood on it. if you were to stand on they acrylic cup it would bend quite a bit and then break.

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Would the cup be half full or half empty and would it make a difference?