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1st timer, 97 6.5 td 2500 14 bolt ff...coming off a dead stop i have no problem, when im running about 30 and above and let it coast then reapply throttle im getting a clunk from out back, also getting a groan under load while turning from stop...rebuild of rearend? U joints?? Any help is appreciated
 

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Welcome to DieselPlace
I would start with U Joints
(y)
 

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You can check your glove box code sticker and see if it at least originally had a limited slip. Code for that was G80. That adds a lot of little flimsy looking parts to the diff that could be contributing to whats going on.

Other questions, is this 4x4? And which 14 bolt do you have? Its a stupid way to refer to these since for 20 some years there were 2 completely different rear ends that were equally popular in these trucks that both had 14 bolt covers. If you don't have any sort of hubcap its really easy just look at whats sticking through the wheel. Giant hub with 8 bolts you have the 10.5 full floating axle. IF its flat with no bolts like a classic RWD car you have the 9.5" semi floating axle.

If you have some sort of covers then you have to bend over and look at the pinion area, the 10.5 has a bolted on plate, and the 9.5 is all part of the main casting. Both had the G80 option. You can't really remove the G80 without changing the whole center carrier so usually the only time that happens is when one eats itself and destroys all the internals anyway. If you don't care about limited slip you can get a standard carrier and if you want it both have various limited slip options available too.

Since its not too difficult or expensive I would get me a gasket, rtv, and some new oil and pop the cover and see what you see. IF its making bad noises and they are coming from inside the diff you don't have to be any kind of expert to see the issues, it will usually be pretty obvious.

If its 4x4 you could have issues in the transfer case making noises but the turning doesn't seem like it would play into that as much.

Another thing to look at while you are down there especially if this truck has any rust on it is look close at your springs and the U bolts where they go through the big plates. Had a few stores here recently where those bolts were rotted through and let the axle slide back and drive shaft fell out on the road. That could go bad real quick and it doesn't have to be an especially salty area since is basically a cup designed to hold crap and moisture.
 

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You can check your glove box code sticker and see if it at least originally had a limited slip. Code for that was G80. That adds a lot of little flimsy looking parts to the diff that could be contributing to whats going on.

Other questions, is this 4x4? And which 14 bolt do you have? Its a stupid way to refer to these since for 20 some years there were 2 completely different rear ends that were equally popular in these trucks that both had 14 bolt covers. If you don't have any sort of hubcap its really easy just look at whats sticking through the wheel. Giant hub with 8 bolts you have the 10.5 full floating axle. IF its flat with no bolts like a classic RWD car you have the 9.5" semi floating axle.

If you have some sort of covers then you have to bend over and look at the pinion area, the 10.5 has a bolted on plate, and the 9.5 is all part of the main casting. Both had the G80 option. You can't really remove the G80 without changing the whole center carrier so usually the only time that happens is when one eats itself and destroys all the internals anyway. If you don't care about limited slip you can get a standard carrier and if you want it both have various limited slip options available too.

Since its not too difficult or expensive I would get me a gasket, rtv, and some new oil and pop the cover and see what you see. IF its making bad noises and they are coming from inside the diff you don't have to be any kind of expert to see the issues, it will usually be pretty obvious.

If its 4x4 you could have issues in the transfer case making noises but the turning doesn't seem like it would play into that as much.

Another thing to look at while you are down there especially if this truck has any rust on it is look close at your springs and the U bolts where they go through the big plates. Had a few stores here recently where those bolts were rotted through and let the axle slide back and drive shaft fell out on the road. That could go bad real quick and it doesn't have to be an especially salty area since is basically a cup designed to hold crap and moisture.
You can check your glove box code sticker and see if it at least originally had a limited slip. Code for that was G80. That adds a lot of little flimsy looking parts to the diff that could be contributing to whats going on.

Other questions, is this 4x4? And which 14 bolt do you have? Its a stupid way to refer to these since for 20 some years there were 2 completely different rear ends that were equally popular in these trucks that both had 14 bolt covers. If you don't have any sort of hubcap its really easy just look at whats sticking through the wheel. Giant hub with 8 bolts you have the 10.5 full floating axle. IF its flat with no bolts like a classic RWD car you have the 9.5" semi floating axle.

If you have some sort of covers then you have to bend over and look at the pinion area, the 10.5 has a bolted on plate, and the 9.5 is all part of the main casting. Both had the G80 option. You can't really remove the G80 without changing the whole center carrier so usually the only time that happens is when one eats itself and destroys all the internals anyway. If you don't care about limited slip you can get a standard carrier and if you want it both have various limited slip options available too.

Since its not too difficult or expensive I would get me a gasket, rtv, and some new oil and pop the cover and see what you see. IF its making bad noises and they are coming from inside the diff you don't have to be any kind of expert to see the issues, it will usually be pretty obvious.

If its 4x4 you could have issues in the transfer case making noises but the turning doesn't seem like it would play into that as much.

Another thing to look at while you are down there especially if this truck has any rust on it is look close at your springs and the U bolts where they go through the big plates. Had a few stores here recently where those bolts were rotted through and let the axle slide back and drive shaft fell out on the road. That could go bad real quick and it doesn't have to be an especially salty area since is basically a cup designed to hold crap and moisture.
It is a 4x4 10.5 full floater...ill grab a gasket rtv n oil, any recommendations for oil...n appreciate the input...my pops bought this brand new and took immaculate care of it...i bought it from him 4 yrs ago. Just rolled 117k so im figuring routine maintenance at this point but ya never know.
 

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You can use what ever oil you want. These didn't require anything special but there is debate if any of the special limited slip additives or oils hurt or not. If you get in there and somehow your dad forgot he had it replaced at some point with some other style then you may need something special but otherwise just your favorite brand of 75-90.

The one Gm was using at the time was pretty unique and easy to identify. Its got a bunch of little gears and springs and sheet metal looking parts all over the place. Really doesn't look like something that belongs in a diff whipping around in heavy oil but overall they worked decent. Most limited slips don't have all that external stuff, its mostly inside the carrier so the center section will be pretty solid looking. IF its got no limited slip at all then the center is really open and you can clearly see and access the spider gears from just about any angle. The limited slips tend to hide them somewhat.
 

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Agree with checking U-joints and/or carrier bearing if 2 piece shaft.
I use gaskets for the covers and not RTV. Makes future maintenance easier/cleaner.

If you have the G80 locking differential the groan could be from sticky clutches.
We've seen different fluids help with this. Sometimes it took draining the factory synthetic and refilling with regular 80w-90.
 

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Check Amazon, there is a seller that sell pre-made silicone gasket which does not need anything to put on.
It is a little pricy but I think it may be worth it.
 

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I use gaskets on stuff like this when I get them which for something like this should not be a problem. I often also use the rtv but not a bead just a thin film. I know there are other products made specifically to work like that but I don't often do enough of this type of work and I always seemed to have a dozen different products on the shelf 3/4 full and dried out from age.

The 'gov lock' that this vintage truck used clutches shouldn't have any pressure on them unless one wheel is actually spinning. This is why this design doesn't use the special fluid. Many of the other designs have pressure all the time so just turning a normal corner causes some slipping to need to occur so that is where the magic is needed. These you have to spin the tire so the governer weights extend and then put pressure to lock the clutches.

If you drive normal on clean dry pavement all the time none of the fancy parts in the system do anything at all. Other systems you can get wear from just normal driving and turning corners. I think what mostly kills these is over speed of one wheel spinning like ice or mud. When you see it you will understand. .
 

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The 'gov lock' that this vintage truck used clutches shouldn't have any pressure on them unless one wheel is actually spinning. This is why this design doesn't use the special fluid. Many of the other designs have pressure all the time so just turning a normal corner causes some slipping to need to occur so that is where the magic is needed. These you have to spin the tire so the governer weights extend and then put pressure to lock the clutches.

If you drive normal on clean dry pavement all the time none of the fancy parts in the system do anything at all. Other systems you can get wear from just normal driving and turning corners. I think what mostly kills these is over speed of one wheel spinning like ice or mud. When you see it you will understand. .
I understand the construction and operation of the gov-loc and actually like it for a tow rig. That said, they can give a nasty slip/stick condition on turns. My 2000 K3500 experienced it with a slide in camper and trailer in tow. You'd of thought a axle shaft was gonna break. Drained factory fill for regular 80w-90 and it went away. Other techs I know ran into similar issues over the years.

Maybe a R&R and check clutches and tolerances would find the culprit but almost 90k miles later it hasn't came back.
 
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