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.

 

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?

 

Maybe an issue with the lube they are using, if it is coming from the same container each time.

 

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.

 

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.

 

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.

 

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.

 

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?

 

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.

 

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.

 

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.

 

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?

 

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.

 

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.

 

Thank you. It's very difficult for me to find out anything about what they find since it's a shop that is doing the work that I have to travel about 60 miles to get to. Twice I had my truck towed there and it got there before I did so I really don't know what they find or what they don't find. When I go to the shop, my family comes with me so we can do something for part of the day while it's at the shop so I can't just hang around to find out what's happening. I'll ask those questions though to see if they have checked those settings previously and if they found anything abnormal or out of whack. I think they would have told me by now if they have since they've said several times that the installation goes totally fine, the gear pattern checks out fine, they've road tested it themselves and it's been okay (although no full throttle testing) and they've checked out the housing as best as they know how. They do around 50 diffs a month so their experience 'shouldn't' be lacking.

Question: what could cause the pinion to move under shock loads like a hard shift?

 

Thank you. It's very difficult for me to find out anything about what they find since it's a shop that is doing the work that I have to travel about 60 miles to get to. Twice I had my truck towed there and it got there before I did so I really don't know what they find or what they don't find. When I go to the shop, my family comes with me so we can do something for part of the day while it's at the shop so I can't just hang around to find out what's happening. I'll ask those questions though to see if they have checked those settings previously and if they found anything abnormal or out of whack. I think they would have told me by now if they have since they've said several times that the installation goes totally fine, the gear pattern checks out fine, they've road tested it themselves and it's been okay (although no full throttle testing) and they've checked out the housing as best as they know how. They do around 50 diffs a month so their experience 'shouldn't' be lacking.

Question: what could cause the pinion to move under shock loads like a hard shift?

That's my point.The pinion shouldn't move if the install and set up was correct. Either the preload was not correct? Maybe the pinion Brg races were not seated all the way in the housing? Did the pinion nut come loose? Did the pinion Brg's start to fail?

Thats the reason for the question's about the preloads. I just don't think you have enough HP to hurt that diff like that.

 

^ Good info there.

I've proposed to the shop that they refund my money and I go elsewhere. They are actually open to this so I might have to return the gears, posi unit and bearings if they want them. I would buy another complete stock rear end I know of that was rebuilt just a year ago with 3.40 gears, take the money I recover and have another shop that is highly regarded do another upgrade with new 3.73 gears and an Eaton.

 

Talked to the new shop. Whew, some of their stuff is 'spensive. Anybody hear of American Axle as a supplier of gear sets? They really like them and said they make the OEM stuff. Checked out their website and they do indeed make a lot of stuff for GM.

Check out this funkiness. I bet that ain't cheap:
http://www.aam.com/index.php?s=49&item=99

They wanted $450 for a gear set from them though. Yukons are like $300, as are Richmonds (but I keep hearing Richmonds are wicked noisy but super durable) and I thought that was pricey.

If I buy a new Eaton elsewhere like Ebay and give up the shops warranty if it fails, I can save like $200. I was hoping to be able to get the money back I was hoping for from the shop ($1500) and be able to buy this other housing and get the new diff rebuilt but after their prices, taxes and buying an Eaton on Ebay I'd still have to pay an extra $200. That would be less than 1/2 though if I paid the other shop to try and fix it again......

 

That seems a little steep!

AAM is good stuff. They can be a little tricky if someone has not done them before. You have to run the backlash tighter than usual and it's amazing how much the pattern will change with a small backlash adjustment.

Do some shopping! Yukon or Precision is good stuff to,and I've had no problems with any of them. Summit/Jegs/ Randys/westcoast/ sixstates/Jt's
just to name a few.

Back to the housing, If the Brg's are not spun in the housing and they claim the gear set got a good pattern,why replace it? If the housing is a problem child it will show up in the assembly and set up.

 

So the shop owner accepted my refund proposal. All I have to do is remove the carrier and gears. Who wants to give me some pointers It's an Eaton limited slip. Do I remove it like a gov-lok or like an open carrier? My Haynes manual describes but not in great detail. Any tips?

I bought a replacement rear diff today. Stock 77 12 bolt with a 3.40 and gov-lok. 5 years ago new gears and a new gov-lok were put in the truck, it was used for a year and then the motor blew up so it was sitting for 4 years. Internals looked fine. It's going to go in my truck after I clean it, paint it and swap over the new brakes I put on my current diff last year. I'll run it like that until early spring and do the 3.73s and new Eaton. My truck hibernates for most of the winter anyway.

 

I took out the rear carrier and ring gear yesterday. That was a struggle getting the carrier out. The bearings did NOT want to pop loose. I used the rag between the gears trick and it got the driver side bearing out a little but the pass side refused to give. After wrenching on the case with a short pry bar I did the rag trick again and the driver side came out a little more again but now the carrier was skewed and maybe even more wedged in. I started hammering on the driver side bearing race (the metal ring around the bearing itself, is that the race?) to maybe pop it back in so I could try and get this thing out straight but then it broke and everything came out nice and easy after that

The pinion is still in though. I thought I could have the wheels on the ground and use that to keep the pinion from moving as I tried to break the nut loose with my breaker bar. Didn't work. Truck would move a little, taking up all my room to move my breaker bar. So, I figure I'll get a big monkey wrench with the longest tail I can find and put that on the pinion flange and wedge it against the garage floor while I use my breaker bar. Sound good?

 

Spread the housing, that's the RIGHT way to do it, that's also why they are equipped with spreader holes facing the rear of the case.

A few well placed welds, and a couple bolts run on a grinder to fit into the holes, as well as a few scrap pieces of angle iron or other decently solid pieces of metal and it should slip right out without any trouble.

 

Well I sure as heck hope the monkey wrench holds on strong enough. Otherwise it's make my own tool out of a piece of angle iron, or try to find a place that will rent me a strong enough impact gun.

 

You don't need a spreader. Just keep the brg races straight while prying the carrier out straight as well. It might make your life easier to use a impact gun.any of your friends have a compressor they can bring over?

 

Oh I got a compressor, I just have a weak impact gun I refer to it more like a bulky air ratchet lol

I managed to get it all out though. The monkey wrench worked great to keep the yoke in place while I used the breaker bar and socket to get the pinion nut backed off. Then I tapped on the pinion with my ball peen and the yoke nudged itself off, which freed the pinion. The diff is completely empty now, save for one pinion bearing.

Now I have to strip the 'new' rear end I bought of it's brakes and wire wheel it, then coat it with chassis saver. I'll swap the brakes over from the rear end in my truck now since they are only a year old, then swap housings.

The diff I bought was rebuilt 5 years ago, and then sat for 4 years. Oil was drained a month ago and I'll put fresh in. Anything I should do to clean it out? Spray down the gov-lok with lubricant oil or brake cleaner to get any gunk out?