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Old 01-08-2007, 12:49 AM   #21
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From my Mechanics of Materials studies....

The shear strength of a grade 8 bolt is in fact higher than that of a grade 5 bolt. Asa general rule, shear strength is typically 60% of the ultimate strength in tension. Since Grade 5 bolts have an 85kpsi tensile strength and Grade 8's are rated at 130kpsi, obviously the Grade 8's have more shear strength (20% greater).

The brittleness argument is one that just doesn't seem to go away. But what is brittleness? Brittleness is a failure at a load below that of at which the object would normally be permanently changed (typically in diameter). Grade 8 bolts exhibit no higher failure of this kind than Grade 5. So what makes a Grade 8 bolt different? Its hardness. Hardness and brittleness are not the same. Hard means it won't bend as easily.....brittle means it has an inexplicable failure.

Ok..that much is settled now. So where does the myth stem from? Well....a Grade 5 bolt is far more likely than a Grade 8 to bend before failure. This is due to the hardness. However, under no circumstances will the Grade 5 bend without failing while the Grade 8 breaks and fails. Case closed.

So...why use Grade 5 bolts? There are several reasons, the first being cost. Grade 8 DOES cost more...significantly in some cases. The second reason is that you just plain don't need the strength of the grade 8. Trailer hitches are a good example of this. The final reason I would use a Grade 5 instead of a Grade 8 is that I WANT the bolt to fail first. It's sort of a shear pin situation, not unlike guys who upgrade ever aspect of an axle except the u-joints because u-joints are cheap...I would rather break a u-joint than an alloy axle if something absolutely has to give.

If you're worried about strength, remember one thing....torque those suckers down! Bolts are designed as clamping devices, not shear devices. If you clamp the hitch to the frame well, the forces will first have to overcome the static (and kinetic eventually) coefficient of friction and the forces resulting from that before the bolt is actually subjected to the shear load. In the case of a hitch where the bolts are in a vertical position, they will ultimately only be subjected to a tension load from clamping the hitch to the frame. The shear load is not passed onto the bolts because of the friction. Grade 8 bolts will have the distinct advantage in this use because they can be subjected to higher tensile loads. So why aren't they used with hitches? Because they aren't needed! The forces aren't such that it would be an issue. The hitch ball in its single shear configuration or the ball mount itself is far more likely to fail.

Thank you, Society of Automotive Engineers.
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Old 01-22-2007, 10:49 PM   #22
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Good post plenty of info its nice to see some numbers not just opinions.

My plan for the hitch on thomas.

I think I wil use a 24" prefab receiver tube, cut a square hole in the rear bumper just above the lower lip, where it can be welded to the bumper at a number of different points. The tube will be installed with the draw bar pin on the inside/forward edge of bumper as a sort of saftey device. From there I think my rear cross member is close enough to allow a spacer block between the tube and the crossmember which will be bolted. Not set in stone but Ill take pics when its done. Right now there is 12" of snow arround the bus at the farm so i will be waiting for a while.
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Old 01-29-2007, 10:43 PM   #23
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Today at the dealership we received 2 new c4500 trucks converted by monroe. They came with a very large hitch measured 35" between the vertical plates that mount to the frame. My price through the dealer is $256. I can get a PN for anybody that wants to call monroe truck for themself
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Old 01-30-2007, 01:03 AM   #24
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I'd like the PN just for GP
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Old 01-30-2007, 10:50 AM   #25
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Monroe truck and equip PN is 00051375
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Old 01-30-2007, 04:57 PM   #26
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I just did an on-line check for the part....No Products Matched Your Search For Part Number "00051375"...
Is there a suffix or prefix to the PN?
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Old 01-30-2007, 05:35 PM   #27
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no thats all they gave me for the PN.
MONROE
1051 W. 7th Street
Monroe, WI 53566
608-328-8127
800-356-8134
Fax: 608-328-4278


JOLIET
812 Draper Avenue
Joliet, IL 60432
815-727-3134
800-892-7052
Fax: 815-727-5429


GREEN BAY
1151 W. Main Avenue
De Pere, WI 54115
920-336-8068
800-848-5400
Fax: 920-336-8118




FLINT
2400 Reo Drive
Flint, MI 48507
810-238-4603
877-233-2030
Fax: 810-238-5009



MARSHFIELD
1601 E. 29th Street
Marshfield, WI 54449
715-387-8445
800-882-1900
Fax: 715-387-3952
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Old 01-30-2007, 08:13 PM   #28
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OK then, thanks anyway
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Old 01-30-2007, 08:28 PM   #29
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The closest PN I can figure is MTE-51037
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Old 01-31-2007, 09:57 AM   #30
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thats probobly the one. Our sytem here at the dealer requires the zeros on all PNs other wise they wont come up in inventory
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