GM Mechanic Question

[Skywalker]

I had my 1970.5 Z28 12 bolt rear end rebuilt by some racer friends of a friend. They removed all the guts, axels, chunk and all and blasted the rearent and then assembled it with my very very slightly used 4:11's which is what the car came with from the factory and is what I was looking for, for a long time. New bearings and all and professional done. However, this is what transpired! It took them a long time as they are in racing season and racing both instate and out of state and just very busy. Rusty told my friend Larry that the rearend was assembled and my buddy Larry thought he was lieing cause Larry from the sandblasting still had my original yoke! We call Rusty and question him and he said, no I built it and used one of my yokes I had laying around here. Well, since my is all original 1970.5 true Z28 stuff he pulled the yoke and came and got my original yoke and put it on. I'm happy and picked up the now assembled and painted with epoxy paint and looking new rearend! I fill with mobil 1 and limited slip additive before I install and sat it in the garage floor on a bed sheet with the intention of installing with my son the following day or so. The yoke peed about a dixie cup of fluid right out the yoke bolt and not from behind the yoke! So, I'm on google because I'm just a shade tree machanic and not a rear end specialist and looking for info. What I think is, when Rusty put his used yoke on and then pulled it after it was torked to specs and then put my used original yoke on and you can bet that with both yokes being used tollerance or wear between the two would not be the same and the crush sleeve is not crushed maybe as much or same as it was before the 1st yoke ws torqued into place. Fluid continues daily and these boys are still busy and I'm impatient and had Impact tools. Cheltons says something like this could be remedy by marking the yoke bolt and pinion splin before he removed the 1st one and then come back and torque just a bit more to be sure and crush the bearing. Can someone with expertease in this are guide me with a proper fix on this issue without paying someone to pull the entire thing. Thanks in advance! Frustraded..

 

[lavinrac]

Well skywalker to me it sounds like you may just be leaking from the pinion/yoke seal. Could be caused by maybe a different diameter yoke that was used so the seal wont seal with the original yoke installed..

But you are correct. if you dont have it torqued down enough the yoke wouldnt be far enough in to get a good seal.. check the seal and the torque specs if you can worse comes to worse a new crush sleeve is pretty cheap.

Not to familiar with the 12 bolt setup.. so it may not even be an issue.

I am sure someone will know more. hope it helps.

 

[lavinrac]

one more thing you might think about if you get to checking specs.. if it isnt torqued down properly and you set it right. The tolerances/backlash may not be correct anymore. Just depends on how he shimmed it. So you may wind up with some play and the dreaded whine.


oh and as a side note.. my sweetheart says to solve rear-end problems eat more fiber.

 

[Skywalker]

I need more figer! I'll get me some medimucial! LOL

one more thing you might think about if you get to checking specs.. if it isnt torqued down properly and you set it right. The tolerances/backlash may not be correct anymore. Just depends on how he shimmed it. So you may wind up with some play and the dreaded whine.


oh and as a side note.. my sweetheart says to solve rear-end problems eat more fiber.

 

[Mr Bogus]

first, if the yoke that was on it fit, you should have left it in place.. that is not a part that is serialized or even "car" specific, they have cap diameters..

IF you want this done right, you do not have a lot of options

The bearing preload is "CRITICAL" and is measured with an inch pound torque wrench.. by hand? it is a firm amount of pressure to turn.. HOWEVER.. you are now turning the gear set with the pinion and that is going to change things..

You are going to need to pull the rear cover, pull the axles and pull the carrier assy and reset the pinion preload (you may likely need a new crush collar too). The intial torque to get the collar compressing is considerable (we use impacts) but after it starts, it goes very fast and it is very easy to "over tighten" the thing.. it takes a skilled hand.. You can NOT just set the torque on the nut holding the yoke on the pinion..

If this is a limited slip or posi rear end, you need specific gear oil AND I would get a bottle of the additive from GM (designed to keep them from chattering, we called it "fish oil")

In short, changing pinions is not a job for the inexperienced.. if you simply reinstall the new yoke? you may or may not have problems down the road.. I suggest you haul the thing back to whom ever did it initially and let them fix it for you..

(hope they checked the pinion depth and gear set pattern too)

 

[SKnight]

first, if the yoke that was on it fit, you should have left it in place.. that is not a part that is serialized or even "car" specific, they have cap diameters..

IF you want this done right, you do not have a lot of options

The bearing preload is "CRITICAL" and is measured with an inch pound torque wrench.. by hand? it is a firm amount of pressure to turn.. HOWEVER.. you are now turning the gear set with the pinion and that is going to change things..

You are going to need to pull the rear cover, pull the axles and pull the carrier assy and reset the pinion preload (you may likely need a new crush collar too). The intial torque to get the collar compressing is considerable (we use impacts) but after it starts, it goes very fast and it is very easy to "over tighten" the thing.. it takes a skilled hand.. You can NOT just set the torque on the nut holding the yoke on the pinion..

If this is a limited slip or posi rear end, you need specific gear oil AND I would get a bottle of the additive from GM (designed to keep them from chattering, we called it "fish oil")

In short, changing pinions is not a job for the inexperienced.. if you simply reinstall the new yoke? you may or may not have problems down the road.. I suggest you haul the thing back to whom ever did it initially and let them fix it for you..

(hope they checked the pinion depth and gear set pattern too)

What he said. While simple, it's not easy. Being the only guy in the dealership that will build a diff is a good thing though.

If the fluid is coming from behind the nut two things come to mind. Either the clearance in the splines is too loose, or it's not tight enough. When I would replace a pinion seal (Same procedure as swapping the yoke.) I would measure the rotational force before removal then add five inch pounds on reassembly, that way I know I tensioned the crush sleeve. On a new diff I would only add a couple of inch pounds.

GM diffs built in the last couple of decades had sealer on the splines and under the yoke washer. If the factory bothered it must help, if done neatly it won't be seen either.

Any way you cut it it sounds like it's time for them to go back and do it again.

 

[shadetree57]

Yep, What Bogus and Orpheus said. You start the build with the correct preload on the pinion bearings. To keep from having a noisy diff or gear failure, better let them reset the pattern.

 

[Skywalker]

Thanks for all the inputs and I have spoke with the builder and he will come and take care of this issue. I will not attempt to do this. I'm sure he know how much preload or torque he was at initially and hope he can do this without a tearing it down. It's unfortunate the yoke was changed after the initial build but what's done is done. These guys build engines and rearends daily and I was just hoping to do a little myself, I'll just have to wait for both these guys has the flu. I had read on google about checking the amount of inch pounds prior to removal and then go back just a hair more. I'm hoping it will all work out!

 

[Zoinks!]

As the others have said, your best option is to get them to set it up again. If it's leaking, then they didn't do something right the first time and need to fix it for you. I would highly recommend a new crush sleeve when they put it back together. That's only one of the VERY critical steps in setting up a rear axle and if it's already out you may as well go back with a new one to eliminate more complications. It's not something for the novice/weekend driveway mechanic. I also suggest they put some liquid teflon on the splines. It'll help make sure you don't get any leakage.

 

[Skywalker]

I'll go over this info with him when he comes to include the crush sleeve and liquid teflon. Many thanks.

As the others have said, your best option is to get them to set it up again. If it's leaking, then they didn't do something right the first time and need to fix it for you. I would highly recommend a new crush sleeve when they put it back together. That's only one of the VERY critical steps in setting up a rear axle and if it's already out you may as well go back with a new one to eliminate more complications. It's not something for the novice/weekend driveway mechanic. I also suggest they put some liquid teflon on the splines. It'll help make sure you don't get any leakage.