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Old 09-28-2009, 02:38 PM
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85 Chevy 1/2-ton 12 bolt rear diff problems

I have a GM truck 12 bolt in my 85 Chevy 1/2-ton shortbed weekend hot-rod.

http://www.cardomain.com/ride/3001366

It's no huge powerhouse with having around 350 horse and runs high 14s with 3.40 gears. The TH350 has what I think is a stage 2 shift kit in it as it shifts very very firm at WOT. Originally it had a 2.76 open 12 bolt that I swapped in a 3.40 Gov-lok (non-working) last summer, a couple months after I had the new cam and heads put in. It ran just fine for a year, including some drag strip outings where I probably put about 20 runs in, plus around-town driving. I haven't had one problem at all with this diff. No noise, nothing.

Here's a quick rundown of the problems I'm having:

- July 2009 I see a local specialty rear-end shop is selling a rebuilt Eaton posi. I buy it to replace the non-working gov-lok
- took it to said shop for install. They had to TORCH out the gov-lok because it was all seized up in there. Put new diff in, all new bearings and seals, axles were said to be fine, R&P was said to be fine, put it all back together
- 5 days later the R&P wipes out after a WOT run through the gears, without any indication or previous noise. Wiped out meaning lots of gear noise and howl
- take it back to shop, they put in Yukon 3.73 thick gears to fit my carrier. I pay parts, they pay labor
- at end of break-in period, I try a WOT run. I get some gear noise. 4 more WOT runs (not at track, just around town) doing 1-2 shifts = wiped out R&P
- take back to shop, they replace gears very reluctantly and after a BBB complaint with new Yukon 3.73 thick gears again. No cost to me. They do not give me any explanation as to why gears failed. They do initial run-in of gears
- after break-in period is done, do an oil change, and first WOT 1-2 shift I do, I get some noise. Not a ton, but some. I live with it for 250 additional miles. I do another WOT 1-2 shift this past Friday. A little more noise but hard to tell. I do another WOT 1-2-3 shift yesterday and I got more noise. At this point I feel doing 2 or 3 more WOT runs will again ruin the R&P.

So, this is where I am. Something I feel is causing the pinion to move under the torque spike during the 1-2 shift and possibly 2-3 shift, mashing it into the ring gear. My thoughts are either:
a) crush sleeve is moving. Why?
b) torching out my gov-lok caused something to tweak. Like what?

If anybody can please help me with this, I would sure appreciate it. I'm going to call the shop back now and report my results. Thanks!!

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Old 09-29-2009, 05:42 AM
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Make sure the axle tubes are firmly attached (read -- still welded) to the center section. The plug welds on many factory axles have been known to break under stress, resulting in strange axle behavior.

Past that, gear tooth pattern and backlash would be my next two things to check. If that all checks out, I'd be pulling the axles and differential to see about the pinion bearing preload and do a full inspection of the case. Like you said, something in there is not the way it was before the 'pro' shop got ahold of it.

Let us know.
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Old 09-29-2009, 11:16 AM
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If the plug weld's came loose,the pinion would've rotated and you would notice that. Sounds like a bad set up to me. Could be either of the following or even a combination.
Pinion depth.
Backlash.
Pinion Brg preload.
Carrier Brg preload.

Is it actually Breaking the teeth or gauling them up?
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Old 09-29-2009, 01:42 PM
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Maybe an issue with the lube they are using, if it is coming from the same container each time.
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Old 09-29-2009, 05:05 PM
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It would seem that perhaps the pre-load is disappearing, but why? Can that be checked without ripping it apart?

The teeth that I saw on the last R&P they pulled out weren't broken but looked like they were contacting each other and worn that way. So I guess you could say galled?

I suspected the lube too. They said they only us GM lube as they've had a chemical engineer analyze it and say it's the best. I had it changed anyway to Castrol Hypoy-C and Ford Motorsport additive. Before I tried the 2nd and 3rd WOT runs, it seemed as though the new fluid quieted down the howl a little bit that came about from the 1st WOT run. But now that I've done the 3rd one, the noise is there and it's louder than ever.

I talked to a reputable shop today and they didn't have a lot to tell me other than to start suspecting the housing. Still though, it was fine when I brought it there. Something changed since they got their hands on it.

Oh, they did say that it's been difficult to source high quality gear sets. Lots of junk is saturating the market. He even said he saw some ITA stamps on Yukon gears (made in Italy, not USA) and then they started having problems with their gears so they won't use them anymore. Too much offshore-made garbage out there. I suspect too if I got two sets of Yukon 3.73 Thick gears from the same bad batch.
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Old 09-29-2009, 05:28 PM
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Quote:
Originally Posted by SUX 2BU
It would seem that perhaps the pre-load is disappearing, but why? Can that be checked without ripping it apart?

The teeth that I saw on the last R&P they pulled out weren't broken but looked like they were contacting each other and worn that way. So I guess you could say galled?

I suspected the lube too. They said they only us GM lube as they've had a chemical engineer analyze it and say it's the best. I had it changed anyway to Castrol Hypoy-C and Ford Motorsport additive. Before I tried the 2nd and 3rd WOT runs, it seemed as though the new fluid quieted down the howl a little bit that came about from the 1st WOT run. But now that I've done the 3rd one, the noise is there and it's louder than ever.

I talked to a reputable shop today and they didn't have a lot to tell me other than to start suspecting the housing. Still though, it was fine when I brought it there. Something changed since they got their hands on it.

Oh, they did say that it's been difficult to source high quality gear sets. Lots of junk is saturating the market. He even said he saw some ITA stamps on Yukon gears (made in Italy, not USA) and then they started having problems with their gears so they won't use them anymore. Too much offshore-made garbage out there. I suspect too if I got two sets of Yukon 3.73 Thick gears from the same bad batch.
Have you pulled this apart yet? The things I would be looking for is:

When the carrier is removed does it just fall out? The carrier Brgs need preload and you should have to work a little to get the carrier out. If the carrier does just fall out look at the carrier shims and look for sign's of the race spinning on the shim. Also make sure the Carrier Brg's have a tight fit and have not spun on the hub.

Check the inner and outer Pinion Brg races. They should have a tight fit when pressed in the housing.

If the oil level was correct I doubt it was a oil problem. Something was missed in the setup or the races or spinning in the housing.

Last edited by prostreet6t9; 09-29-2009 at 08:56 PM. Reason: Spelling
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Old 09-29-2009, 08:07 PM
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Bad gears is possible if they are from the same source. I have used hundreds of sets of Yukon gears and have only had issue with a couple. Both GM 8.5 and both were not heat treated properly and wore down to nothing in a few hundred miles. Dont know what else it could be thats not obvious on assembly.
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Old 09-29-2009, 09:01 PM
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If it hasn't been pulled apart yet,check the pinion yoke and see if it moves in and out. There should be NO movement in and out.
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Old 09-30-2009, 10:37 AM
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Thanks guys. I talked to the shop yesterday and they were at a loss, as am I, but they said it might just be the carrier I bought from them and had installed originally. That's what started this whole process. I bought an old rebuilt Eaton Posi-trac unit from them that they rebuilt. It's the same style as a Top-spec but what was OEM before the Gov-lok came out. Anybody know what I am talking about? Anyway, they did say when I bought that the cases on those are thin but it should hold up okay for my application. They feel now that it isn't. They even swapped the one I bought for another one last time it was in for a new R&P. I didn't know that until yesterday. They feel that it's allowing ring gear movement during the torque spike at the shift points. They didn't say if the previous one was cracked or not though. They want to put in a Top Spec AND change the housing, since I had asked them if the housing could have been damaged when they torched out my gov-lok and they said "I really don't know." I'd really rather not change the housing since that's more cost, more time and I put all new brakes in when I put that housing in my truck last year and painted it pretty gloss black with Chassis Saver.

He said he'd foot the cost for all labor and new R&P (new OEM). I'd have to pay for small parts (fluid, seals, etc), cost difference between the old Posi and the Top Spec and possibly a housing if I really need one. His price though for a Top Spec is $100 more than I can buy one for so that's bunk but he has this policy of no customer-supplied parts. And I can get one off Ebay for $200 less than his price. Gonna try to negotiate on that.

What y'all think?
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Old 09-30-2009, 11:13 AM
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I don't think torching out the GovLok would damage the housing, if all they did was cut the pin and the block in a couple places to get the c-clips out.


The fact that they already changed the diff again without any improvement seems to discount that idea. The factory case withstood 350hp so I don't think any after-market case would have a problem. I'm not sure what a Top Spec is. Is that the posi made by USGear?

I would try and take on of the bad gears and have it Rockwell tested in a couple places and then check your old gears for comparison.
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Old 09-30-2009, 12:08 PM
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A Top Spec is the aftermarket Eaton limited slip unit. I'm using that to refer to an aftermarket Eaton since I've had two Eaton-built units in there already (Gov-lok and then a Posi-trac) and will now probably upgrade to the Top Spec.

It wasn't something I really considered being the weak link, but it's entirely plausible since it's not the upgraded, heavier duty design of the modern Top Spec, it was not in there before I had these problems, GM for some reason replaced it with the then-new Gov-lok back in 1973 when the new design of trucks came out (and were putting in gov-loks across the board I think then too in all their cars, perhaps due to failures?) and the other parts in there seem to be quality pieces, and the shop has a very good rep for knowing what they are doing. Something is causing movement in my diff and I could see a weak carrier doing it.

GM still puts Gov-loks in new trucks (fabled G80 option) and tout how good it works. You can see videos on youtube about it. I still think they are weak and haven't heard of them ever being strengthened. I wouldn't ever trust one in a performance application. They work at low-speeds if you need the tires to lock but are easily overwhelmed.

The previous 3.73 set was sent back to Yukon for warranty and this set will probably do the same. I don't know if the previous gears were found at fault or not for hardness.
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Old 09-30-2009, 12:12 PM
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Quote:
Originally Posted by SUX 2BU
Thanks guys. I talked to the shop yesterday and they were at a loss, as am I, but they said it might just be the carrier I bought from them and had installed originally. That's what started this whole process. I bought an old rebuilt Eaton Posi-trac unit from them that they rebuilt. It's the same style as a Top-spec but what was OEM before the Gov-lok came out. Anybody know what I am talking about? Anyway, they did say when I bought that the cases on those are thin but it should hold up okay for my application. They feel now that it isn't. They even swapped the one I bought for another one last time it was in for a new R&P. I didn't know that until yesterday. They feel that it's allowing ring gear movement during the torque spike at the shift points. They didn't say if the previous one was cracked or not though. They want to put in a Top Spec AND change the housing, since I had asked them if the housing could have been damaged when they torched out my gov-lok and they said "I really don't know." I'd really rather not change the housing since that's more cost, more time and I put all new brakes in when I put that housing in my truck last year and painted it pretty gloss black with Chassis Saver.

He said he'd foot the cost for all labor and new R&P (new OEM). I'd have to pay for small parts (fluid, seals, etc), cost difference between the old Posi and the Top Spec and possibly a housing if I really need one. His price though for a Top Spec is $100 more than I can buy one for so that's bunk but he has this policy of no customer-supplied parts. And I can get one off Ebay for $200 less than his price. Gonna try to negotiate on that.

What y'all think?
Unless the Brg's are spinning on the hub,I doubt it's the carrier. Still seem's like a set up problem.
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Old 09-30-2009, 05:14 PM
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But the gears operate just fine under load after breaking them in, just under WOT when shifting is when they contact. How could that be a setup issue?
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Old 09-30-2009, 05:28 PM
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If setup properly with quality parts, a 12BT with 350hp should go forever. I dont think you can hurt the gears with that amount of power, regardless of which carrier you're using. Even a GovLok, or GrenadeLok, will withstand that for a long time, and they changed the diff out on the second try anyway. A F9 behind 1200hp and slicks will beat down a set of gears in 150 passes. Sometimes less.

If the shop does diff work regularly with good results, I would go back on the gears or the lube.
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Old 09-30-2009, 07:47 PM
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Quote:
Originally Posted by SUX 2BU
But the gears operate just fine under load after breaking them in, just under WOT when shifting is when they contact. How could that be a setup issue?
The reason I think it's a set up issue is because it happened twice. I guess it's possible it's a bad batch of gear set's,but yukon is quality stuff. I use yukon stuff on a daily basis and just don't have that problem. I get into situations all the time where a shop sets up gears and the set up is wrong. They always blame the gear set.

How bad is your gear set tore up? If the gear set has exessive premature wear it will be impossible to check the set up. About the only things you can check is, Does it still have any Pinion Brg preload? Where the Carrrier Brg's still preloaded? If those preloads are off either a Brg has started to fail or the preloads were never right to begin with. So, If you inspect the Brg's and cannot find enough wear to warrant the preload problem it was never set up correctly.

Another thing you can check is the amount of pinion shims that are installed. If the amount of shims is way off,that would also point to the set up.
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