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Old 04-10-2014, 10:22 PM   #1
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Pin Box Bolts - Quantity and Grade

I've looked at 100's of fifth wheels over the years, and noted the way each pin box is mounted. I find it interesting that some manf use 8 bolts, and some use 10 on the pin box. I never understood why they would use only 8, when there are 2 more holes ready to accept bolts in most occasions. I also never understood why they used grade 5 bolts. Maybe from an engineering point of view, that's all you need. My SB came with 8 grade 5 bolts. I guess to give myself the warm fuzzy feeling, I changed them all out for grade 8 hardware, and added 2 more bolts for a total of 10.

Now, at least if my lippert frame cracks or breaks around the pin box, the pin box will stay attached to the piece that breaks off. Yes, being sarcastic, but it's happened too many times as you'll read on this forum.
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Old 04-10-2014, 11:39 PM   #2
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Not likely, but its possible the g5 bolts are used due to g8's being more brittle? Doubtful that's the reason, but they are.

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Old 04-11-2014, 10:04 PM   #3
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Maybe so. I know they put grade 8 on the level up jacks, but they don't see the shock that the pin box does. I think the grade 8 on the pin box will be fine.
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Old 04-11-2014, 10:10 PM   #4
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Maybe so. I know they put grade 8 on the level up jacks, but they don't see the shock that the pin box does. I think the grade 8 on the pin box will be fine.
I'd agree completely. Good to add the extras like you did too. I'll be checking mine.

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Old 04-11-2014, 10:22 PM   #5
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It is all about the right tool for the job. Grade 5 will stretch before they break. That makes them more suited for the pin box due to the "shock loads" the pin box in likely to see. Yes, the grade 8 have a stronger tensile strength, but when they fail, they shear. Shearing is a bad thing when it is the hitch! In the case of the Grade 5 bolts, when the start to fail, they bend/get misshapen, but at least the hitch it is still connected. Hopefully you would notice the misshaped bolt(s) before they failed. With Grade 8, looks good one day, sheared the next. Ten vs eight probably comes down to rated load. No reason to not do ten, other than RV manufacturers are cheap!
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Old 04-12-2014, 10:44 PM   #6
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Good to hear all the input on Grade 8. I'm going to leave them in there and take my chances. I just don't see how 10 - 5/8 inch grade 8 bolts would shear with 2600 pounds on them, but it's possible I suppose. I figure the lippert frame is the weak point on this rig. With my luck so far, I'm waiting for a cracked frame around the pin box any month now.
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Old 04-12-2014, 10:47 PM   #7
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Good to hear all the input on Grade 8. I'm going to leave them in there and take my chances. I just don't see how 10 - 5/8 inch grade 8 bolts would shear with 2600 pounds on them, but it's possible I suppose. I figure the lippert frame is the weak point on this rig. With my luck so far, I'm waiting for a cracked frame around the pin box any month now.
I'm kind of sure the welded frame would cave before the pin box bolts sheared.
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Old 04-12-2014, 11:04 PM   #8
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The gr 8 bolts are much better for shear loads than gr 5 and the pin box is strictly in shear. Good choice. Just make sure you torque them to the proper torque and DO NOT OVER TORQUE them.

Shear is normally 60% of tensile strength. Gr 5 has a tensile strength of 120,000 lb min. Gr 8 has a tensile strength of 150,000 lb min.
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Old 04-12-2014, 11:06 PM   #9
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I added AN hardware to my pin box, I did not like the threads in the hole and or extra washers on the head and nut to get the correct grip. I sized my according to the grip length, plus the QTY control of SAE bolts ae something to be leary of.
AN (aircraft) fasteners are manufactured to rigorous quality control standards from raw material to final plating
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Old 04-12-2014, 11:15 PM   #10
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In regards to torque, what is the proper torque for the 5/8 inch Grade 8 bolt, non-lubricated? Right now, they are at 150 ft / pounds, which is probably a shade low. I've seen specs from 150 to 230. I staged them up to 150 via hand tight, wrench tight, 50, 100, 125 then 150. If they need to be a shade higher, not a big deal. All were torqued with a Snap-On torque wrench, so not a cheapo by any means.
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