U-bolt application for stock

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Steve Addy

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Hey all
I searched and didn't see anything related to my question, which is what size u bolts apply to C3500?

Because there's a lot of confusion in parts houses between 3500HD and just 3500 (DRW / SRW etc) the fitment guide is not clear, but the following is what I think is out there, and that's kinda based on what I see Husky putting out there via RA, but also casual observation on a lot of roached out JY trucks.

C/K1500 (C=5x, K=6x) 9/16" thick
C/K2500 (6 bolt light duty) 9/16" thick
C/K3500 (8 bolt HD) 5/8" thick.

This is kinda what Husky is putting forward and I agree it makes sense although I will say looking at a lot of different trucks in the JY I can't say for certain that GM installed 5/8" bolts on the HD2500 trucks (or 3500), my evidence kinda says that's not true. In fact referencing my own C3500 it doesn't appear that the rear u-bolts are 5/8", they look to me like the 9/16" bolts on my K1500 that I just replaced TBH. Maybe there's another qualifier for the 5/8" bolts I'm not aware of? Like for only trucks with the 9200 GVWR?

If anyone has the skinny on bolt fitment I'd be interested in knowing how that all worked out.

I'm going to be replacing them sometime in the future, probably not this winter, and 9/16 or 5/8 it doesn't really matter but I would probably default to the larger just because.

This doesn't address length which I'm not going into here because that's not a problem really.

Any info or suggestions please post back...

Thanks

Steve
 

stutaeng

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I helped my brother swap a 14 bolt 6 lug into his '90 a few days ago. I think he said the new ubolts he ordered were 9/16" diameter.

I've got a K3500 SRW (GVWR = 9200#) that I can check latter on. If I had to guess, I'd say they are also 9/16"... Isn't your C3500 also 9200#?

I'm not sure what they used for design. I seem to recall these bolts are TTY, but on the instructions my brother got they have some torque specification. Something like 70 lb-ft, but can't remember exactly. Maybe they used different grade of steel, with a different tq. spec, effectively putting more clamping force? Dunno.
 

stutaeng

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Um, I just measured mine and they measured 0.588"...that seems to match 9/16".

I measured the smooth part as indicated by the arrow on the photo.

For reference it's a '00 K3500 CCLB SRW with 5.7.

Interestingly, the Haynes manual has a higher value for torque on U bolts for the 7.4 and/or DRW, so perhaps those got the 5/8" bolts?

That would seem logical. A larger diameter TTY bolt would require a larger torque spec, assuming same yield strength.

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Steve Addy

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Hey guys, thanks for responding...

I helped my brother swap a 14 bolt 6 lug into his '90 a few days ago. I think he said the new ubolts he ordered were 9/16" diameter.

I've got a K3500 SRW (GVWR = 9200#) that I can check latter on. If I had to guess, I'd say they are also 9/16"... Isn't your C3500 also 9200#?

No, mines 9000GVWR but it does have the JB8 hydroboost with 13x3.5" rear drums, 6.5 TD

I'm not sure what they used for design. I seem to recall these bolts are TTY, but on the instructions my brother got they have some torque specification. Something like 70 lb-ft, but can't remember exactly. Maybe they used different grade of steel, with a different tq. spec, effectively putting more clamping force? Dunno.

I don't think they're TTY, the torque required, even with the retorque after 50 some odd miles isn't enough to stretch anything.

Can't say 3500HD, that is a whole 'nother animal....

Oh yeah, the 3500HD that's completely different beast, I think those u-bolts are even curved on top but I haven't looked at many of those.

Um, I just measured mine and they measured 0.588"...that seems to match 9/16".

I measured the smooth part as indicated by the arrow on the photo.

For reference it's a '00 K3500 CCLB SRW with 5.7.

Interestingly, the Haynes manual has a higher value for torque on U bolts for the 7.4 and/or DRW, so perhaps those got the 5/8" bolts?

That would seem logical. A larger diameter TTY bolt would require a larger torque spec, assuming same yield strength.

That's interesting, and thanks for providing that measurement. It's consistent with what I've found on a lot of trucks out there.

I did a casual measurement of mine today since I was replacing the drum backing plate and had that all opened up, and with just a tape measure it appeared to measure 5/8" but I will check again tomorrow with something more substantial. The other part of that though is that there's enough corrosion present that it's just really 9/16 with rust added.

Regardless, I will probably replace with the 5/8" Husky since they're readily available, and based on what I saw today the sooner they're replaced the better....

Thanks

Steve
 

Schurkey

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Let's keep in mind that the U-bolt diameter itself may vary--9/16 vs. 5/8, for example--but the overall width (the "inside diameter") is also different depending on which axle is installed.

The axle tubes on the 1500 (8.5" ring gear) is smallest. The light-duty 2500 9.5" ring gear axle tube are bigger, need a larger spacing on the U-bolts, and the formed-steel mounting plate at the bottom that the nuts tighten against has wider-spaced holes.

I'd expect that the heavy 2500 and 3500 10.5 axle tubes are bigger-still--but I don't know that for sure. And maybe the big-dog 11.5 axle is bigger again.


GM specifically warns to replace the U-bolts each time; don't re-use them. Given the way mine corroded, I understand. But if they haven't corroded, GM still expects you to replace them.

Photo 1. http://hbassociates.us/K1500_Axle_01.jpg
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stutaeng

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Wow, that necking looks aweful! Are you in rust belt area? On the one we removed recently we sheared one of the bolts. Clean shear, just snapped, using a 24" breaker bar.

OP, I didn't realize that the torque is small compared to the diameter. Like head bolts are torqued much higher and they are way smaller diameter bolts.

What year is your truck? I thought all 3500 SRW were 9200#, but figured maybe earlier ones were slightly different? 200# is not a lot anyways.

Going to larger hardware won't ever be a bad idea, so no harm there.
 

Steve Addy

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Let's keep in mind that the U-bolt diameter itself may vary--9/16 vs. 5/8, for example--but the overall width (the "inside diameter") is also different depending on which axle is installed.

The axle tubes on the 1500 (8.5" ring gear) is smallest. The light-duty 2500 9.5" ring gear axle tube are bigger, need a larger spacing on the U-bolts, and the formed-steel mounting plate at the bottom that the nuts tighten against has wider-spaced holes.

I'd expect that the heavy 2500 and 3500 10.5 axle tubes are bigger-still--but I don't know that for sure. And maybe the big-dog 11.5 axle is bigger again.

I think you're right in this case, but to what extent I don't know. I have some good 'plates' or carriers as I call them from a K1500 and compared them to the ones on the C3500 a few days ago and the spacing is just a tad wider fore and aft. I would guess though that the 9.5 SF and the 10.5 FF axles use the same but beyond that I cannot say since I haven't looked at the 11.5 with the idea of investigating the issue.

GM specifically warns to replace the U-bolts each time; don't re-use them. Given the way mine corroded, I understand. But if they haven't corroded, GM still expects you to replace them.

Yes, a lot of manufacturers say to replace a lot of things, in the VW world the word replace comes up for every fastener despite the fact that very few are TTY or 'stretch' except crank timing sprocket and head bolts. Typically this has to do with factory applied loctite of some sort, but rarely if ever do people replace the fasteners, mostly because fasteners sold via auto makers are in many respects unreasonably priced, but also because I can dab a bit of loctite back on it if it really matters. I reuse anything that isn't specifically TTY or stretch and is undamaged, but I can't say that I would for u-bolts that have been installed for a while...too much labor investment.

In the real world people just don't do this despite the warnings, and TBH auto makers do this for two reasons, 1) so that if there's a litigation issue they can deny responsibility and 2) as a side business it can make money when people who don't do their own maintenance come in for significant dealer work, say a rear differential replacement, they can sell to them 4x axle u-bolts as a matter of 'shop practice' because they say so. It's also easier to remove if you just cut them off...

In this day and age I take very little at face value...there's a reason people tell you something...people need to figure out what the reason is and why.

That's not to say that they shouldn't be replaced if they need to be, as demonstrated in your photos and the ones I've added at the bottom. But I will say that on my K1500 I stumbled upon another in the JY that had brand new Husky u-bolts and so I 'relieved' that truck of those and installed them on mine (pics below). I don't believe there's any reason they shouldn't be reused in this instance, who ever put them on didn't tighten them to a level that I think they should have been...in fact they were almost finger tight.

But aside from stumbling upon new products with the labels still on them...reuse of u-bolts is really a huge labor effort I couldn't get behind...


Wow, that necking looks aweful! Are you in rust belt area? On the one we removed recently we sheared one of the bolts. Clean shear, just snapped, using a 24" breaker bar.

OP, I didn't realize that the torque is small compared to the diameter. Like head bolts are torqued much higher and they are way smaller diameter bolts.

What year is your truck? I thought all 3500 SRW were 9200#, but figured maybe earlier ones were slightly different? 200# is not a lot anyways.

Going to larger hardware won't ever be a bad idea, so no harm there.

The torque spec for the u-bolts is nil really 70 lbs first tightening, but must be rechecked after 50+ miles for settling in and shifting, which makes sense. Personally I probably torqued mine more than that....

My truck is 1993 C3500, but the door jamb sticker clearly states 9000# on it. I think there was some other rpo code it needed to be 9200# but I can't remember right now what that was. I will see if I can find my notes from before I bought it, I might have jotted it down at the time. I think it had something to do with rpo 8S9...but it may have been more than that.

Originally the truck was represented to me as having a 'bad rear axle' which I initially scouted around to try and find for this truck, and that's the reason the 9000# v 9200# issue came up, because Hollander makes a distinction between the two. After a month of waiting I forced the issue of purchase with the salvage yard who had it, because it only had 100,000 miles on a 6.5 vin F engine and he told me the story about the prior owner having the wheel fall off on his kid...which made sense with the drum on that side missing and the backing plate destroyed at the bottom...but enough of that hilarious story for now. At any rate they junked the truck....and now I have it and am trying to catch up on all the deferred maintenance and repairs.

Anyway, back to u-bolts, below is my 94 K1500 6.5TD with the new bolts (top) and then the old stuff below. The lower carriers had to be replaced (same on the C3500 too but not as bad as this) but you can see the rust belt condition here.

I broke them all off except one where the nut came willingly with just an 18" breaker bar, that's how bad they were. Of course there's really nothing left of them at this size.

I am glad that a friend suggested I take a look at them. He said a friend of his with a plow truck had the rear end come out from under his last winter...wasn't good and that's something I prefer to avoid...

The third photo shows the one that broke after I changed a tire out...not good.

Thanks for all the commentary and info guys, it really does help out!!

Steve



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Steve Addy

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Measured the u-bolts on the C3500 today, after scrubbing with wire brush caliper read 14.78mm, which is greater than 9/16" but less than 5/8", which makes sense since they're corroded some and in bad shape.

Also, found this online thought it was worth posting up. It comes from Husky but I would think it's probably appropriate. They also say that you should retorque after 20 miles and again every day until the bolts quit losing torque value.

I need to go back and check the K1500 bolts before driving it again.

Steve

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CrustyJunker

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I put some 5/8" u-bolts on my K1500 this year. I'm sure I'll break my 10-bolt before these things let go.

I had to re-torque 4 times because of lost tension. They weren't so loose that they were dangerous, but probably a good idea if you have a long trip coming up or towing to do.

I also had rusty leaf springs that probably had to crunch and rub the old rust out after some cycling. It's rock solid now. :waytogo:
 
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